Cleon Catsambis

Cleon Catsambis

Call: 2011

"Very responsive and detailed in his approach."

- Chambers & Partners UK Bar (2022)

"He works hard and spots the real nub of a case."

- Chambers & Partners UK Bar (2022)

"Cleon in expert in his field who is able to deliver advice pragmatically in uncomplicated language."

- Legal 500 UK Bar (2022)

"A charismatic and hard-working junior. One to watch."

- Legal 500 UK Bar (2022)

"Hard-working, intelligent and always clear about what he is doing. He provides practical and intelligent answers to complicated issues. On every level his instincts are amazing."

- Legal 500 UK Bar (2022)

Practice Overview

Cleon has a strong and established commercial practice that encompasses commercial litigation and dispute resolution, banking & finance, financial services, regulatory & disciplinary law, company law and insolvency. He also has expertise in the areas of aviation, art & cultural property, energy and administrative & public law.

Cleon is recognised as a Leading Junior by both Chambers & Partners and Legal 500 in multiple practice areas and is described as “a highly polished advocate and deep strategic thinker who provides a quality service for all his clients”, “very clever, knowledgeable and easy to deal with”, “an expert in his field who is able to deliver advice pragmatically in uncomplicated language” and as someone who “works hard and spots the real nub of a case”.

He regularly appears as sole and junior counsel in high-value and complex disputes in the High Court, the Court of Appeal and a variety of Tribunals. He also undertakes pleading and advisory work.

Cleon is an editor and contributing author to several leading publications, including Atkin’s Court Practice (Financial Services and Administrative Law volumes), Jordan’s Regulatory and Disciplinary Proceedings and Butterworths Financial Regulation Service. He is also an advocacy tutor for The Honourable Society of Lincoln’s Inn.

Cleon undertakes advisory and pleading work and represents clients in all types of commercial cases, from simple contractual disputes to high-value and complex multi-jurisdictional litigation.

He has particular experience of banking & financial services, fraud and professional negligence claims, as well as applications for interim relief including injunctions, freezing orders, search orders and general matters relating to disclosure and legal professional privilege.

Cleon’s cases often involve an international dimension. He has substantial experience of jurisdictions in the Caribbean (including BVI, the Cayman Islands, the Bahamas, Barbados and Jamaica) as well as the Russian Federation, Cyprus and the Netherlands.

As junior counsel, recent notable work includes:

  • Acting for a custodian defendant in worldwide $2 billion litigation brought by the Danish tax authority to recover withholding tax refunds (with Andrew Onslow QC).
  • Acting for Sir Frederick Barclay and Amanda Barclay in proceedings arising from the covert bugging of their confidential conversations at the Ritz Hotel London, including successfully obtaining a series of ‘doorstep delivery up’ orders, forensic imaging orders and pressrestrictions whilst these were being executed (with Hefin Rees QC): Barclay v Barclay [2020] EWHC 1179 (QB); [2020] EWHC 1180 (QB); and [2020] EWHC 1911 (Ch).
  • Advising, in respect of both domestic and international proceedings, on a US$200m claim involving allegations of transactions at an undervalue and unlawful means conspiracy (with Hodge Malek QC).
  • Acting for Cornwall Council in successfully obtaining declaratory relief that it was entitled to lawfully terminate a 10-year £160m public procurement outsourcing contract forthwith on the basis of material breach (with James Ramsden QC): BT Cornwall Ltd v Cornwall Council and others [2015] EWHC 3755 (Comm).
  • Acting for Japanese smart card chip producer Renesas in a combined follow-on and standalone damages claim brought in the High Court by Vodafone pursuant to the European Commission SmartCard Chips cartel decision.
  • Acting for a prominent Russian businessman in a US$60m Commercial Court claim involving complex issues of trust instrument interpretation, rectification, valuation and Russian and Cypriot law (with James Ramsden QC).

As sole Counsel, recent notable work includes:

  • Successfully obtaining summary judgment to correct a unilateral mistake in a Sale and Purchase Agreement which entitled the claimant to an uplift payment in respect of the onward sale of the business: Ffrees Family Finance Ltd v U Holdings Ltd [2020] EWHC 1911 (Ch).
  • Advising a major broadband and telecommunications provider in respect of a prospective benchmarking claim valued at £100m.
  • Acting for a performance marketing company in a claim for unpaid commission in respect of crypto-currency web traffic generation.
  • Advising in respect of the image rights of major Premier League footballers.
  • Acting for a financial advisory company is successfully obtaining judgment in the amount of £1.3 million in respect of an unpaid transaction fee.
  • Acting for a corporate finance firm in respect of a disputed success fee and equity and loan note agreement (c. £1.2 million) following the completion of an investment transaction.
  • Advising investors in a currency movement algorithm in relation to an alleged fraud of their funds (where the IP of the algorithm is valued in excess of $20 million).

Cleon also has experience of international arbitration, most recently advising in relation to a US$130m claim for breaches of a Shareholders and Noteholders Agreement in the Hong Kong International Arbitration Centre under UNCITRAL rules.

Cleon is recognised as a Leading Junior in Financial Services Regulation by the legal directories and regularly advises and acts for firms, investors, banks and regulators on the full range of issues arising under the domestic and European/international legal framework, including compliance and enforcement, judicial review, professional negligence, mis-selling claims and collective investment schemes.

In 2014, Cleon worked in-house with the Enforcement and Financial Crime Division of the Financial Conduct Authority, concentrating on consumer credit issues. He advised the FCA on the interpretation and application of their statutory powers and considered the legal dimension of applications for authorisation. Cleon has extensive knowledge of the statutory and regulatory regime and can advise on all aspects of compliance, authorisation, supervision and enforcement. He has made written and oral representations and appeared before both the RDC and the RTC.

Recent notable work includes:

  • Successfully representing the Financial Ombudsman Service in a significant judicial review challenge concerning its jurisdiction to review the manner in which an FCA review is conducted (as sole counsel): R (on the application of Mazarona Properties Ltd) v Financial Ombudsman Service [2017] EWHC 1135 (Admin).
  • Successfully overturning a proposed Prohibition Order against a prominent accountant in the FCA’s Regulatory Decisions Committee (as sole counsel).
  • Acting for a group of companies operating in the financial and legal services sectors in the FCA’s Regulatory Transactions Committee in respect of a proposed acquisition which is opposed by the FCA (as sole counsel).
  • Advising a consortium of high-profile Premier League footballers and managers in a claim concerning the collusion of banks, accountants, independent financial advisers and agents in the mis-selling of financial products and investment schemes (with James Ramsden QC).
  • Advising a major Caribbean bank in respect of a US$300m claim against is former directors and senior managers for breaches of fiduciary duty and negligence (relating to mismanagement of the bank’s loan book and regulatory breaches) and against its former auditors for professional negligence (with Hefin Rees QC).
  • Advising on the proper application of the bank resolution provisions under Directive 2014/59/EU in the context of the Republic of Cyprus’ resolution of the Cypriot branch of FBME Bank Ltd on the basis of allegations of money laundering and funding of terrorism (with Hugh Mercer QC).
  • Defending contribution and negligence claims relating to an action under section 90 of the Financial Services and Markets Act 2000 for compensation for losses of c.£23m allegedly caused by misleading information in a rights issue prospectus (with Andrew Tabachnik QC).

Cleon has had extensive involvement as a s.166 FSMA skilled person in a major review into the mis-selling of interest rate hedging products by two high-street UK banks. He has advised on and acted in respect of numerous multi-million pound mis-selling claims.

Cleon is an editor of Atkin’s on Financial Services and of Butterworths Financial Regulation Service and co-authored the chapter on financial services in Jordan’s Regulatory and Disciplinary Proceedings.

Cleon has particular knowledge and experience in company law and insolvency matters and regularly acts for office-holders, companies, directors, creditors and debtors. He has acted in major shareholder disputes, unfair prejudice petitions, derivative claims and litigation concerning corporate governance and directors’ duties.

Recent notable work includes:

  • Acting for Edgeworth Capital (a Robert Tchenguiz vehicle), in a successful bankruptcy petition against Mr Glenn Maud, where the underlying debt related to the financing and ownership of the ‘Financial City’ complex in Madrid (valued at c.£3 billion) (with Jonathan Nash QC): Maud, Re [2020] EWHC 974 (Ch).
  • Acting for the Claimant in respect of a £120m claim against a Cayman firm contracted to deliver governance and director services relating to the Axiom Legal Financing Fund (with Hefin Rees QC).
  • Obtaining an injunction restraining the presentation of a winding-up petition relating to an undisputed debt on the basis of a genuine and serious multi-million pound cross-claim (as sole counsel): LDX International Group LLP v Misra Ventures Limited [2018] EWHC 275 (Ch).
  • Acting for joint administrators in recovering uplift consideration in respect of an onward sale of the business (as sole counsel).

Cleon specialises in regulatory law, particularly in the financial services and healthcare sectors. He has considerable experience advising and acting for regulatory and professional bodies, government departments, firms and individuals.

Cleon is a contributing author to Jordan’s Regulatory and Disciplinary Proceedings.

Financial Services

Cleon practises in all areas of financial services regulation. For further details please see the Banking & Financial Services section of his profile.

Regulation of the Legal Professions 

Cleon acts for both individuals and the regulators in the legal professions.

He has advised and acted for the Solicitors Regulation Authority, particularly in respect of its powers to compel disclosure under s.44B of the Solicitors Act 1974 and on the prospects of success of subrogated claims. He has also advised individual solicitors during investigations by the SRA.

Cleon advises barristers in respect of their professional and regulatory duties, including the duty to self-refer.


Cleon regularly advises and acts for most major regulatory bodies including: the Health and Care Professions Council, the General Medical Council, the General Dental Council and the General Optical Council.

Cleon has extensive experience of statutory appeals to the High Court from the decision of healthcare regulators, see for example: Bryce v Health and Care Professions Council [2018] EWHC 3662 (Admin); Subner v Health and Care Professions Council [2018] EWHC 3906; Estephane v Health and Care Professions Council [2017] EWHC 2146 (Admin); Hall v Health and Care Professions Council (CO/17/2017); and Falodi v Health and Care Professions Council [2016] EWHC 328 (Admin).

Cleon was involved in the judicial review concerning the national re-organisation of paediatric cardiac surgery centres (R (Save our Surgery Ltd) v Joint Committee of Primary Care Trusts [2013] EWHC 439 (Admin)).

In 2012, Cleon undertook a secondment at Monitor, the regulator for NHS Foundation Trusts. He advised on the provider licence conditions, the enforcement regime and statutory interpretation.

Other regulatory experience

Cleon has also:

  • Advised a regulator as to whether its procedures breached Article 6 ECHR and/or Article 1 of the First Protocol;
  • Advised a regulator on the proper construction of its statutory remit as to sanctions; and
  • Assisted in Shell’s successful challenge to the OFT’s finding that it participated in price-fixing in the market for tobacco products contrary to domestic and EU law: Shell UK ltd v OFT [2012] Comp  AR 61.

Cleon is recognised as a Leading Junior in Aviation by the legal directories and described as “an expert in his field who is able to deliver advice pragmatically in uncomplicated language” (Legal 500, 2021).

He has extensive knowledge of and experience in the aviation & aerospace sector. He regularly advises and acts for the Civil Aviation Authority (CAA), airports (including Gatwick Airport, London City Airport, Luton Airport and Edinburgh Airport) and airlines.

Cleon acted for the CAA in the judicial review proceedings concerning its duty to consult in respect of alterations to air traffic control patterns on the Westerly approach to Gatwick Airport (with Gordon Nardell QC): R (on the application of Barraud) v Civil Aviation Authority [2015] WLUK 992.

He has also advised the CAA in respect of airspace changes, consultation duties and post implementation reviews in respect of Gatwick Airport, London City Airport, Luton Airport and Edinburgh Airport.

Cleon has also advised many major international airports on, among other things, the lawfulness of consultation exercises, environmental impacts, trials of new SIDs and changes in airspace design. He has guided multiple change sponsors through all aspects of the airspace change process.

Cleon was involved in the litigation concerning Ryanair’s minority shareholding in Aer Lingus and whether it complied with EU and domestic merger regimes.

Cleon is recognised as a Leading Junior (Tier 1) in Art and Cultural Property and described as “very clever, knowledgeable and easy to deal with”. He has acted for art owners, art brokers, art dealers and leading auction houses in proceedings and regularly advises in relation to art matters.

Recent notable work includes:

  • Acting for a private collector in a dispute as to the ownership and provenance of a one-of-a-kind pre-Islamic jar valued at c.£12m (with James Ramsden QC): Jeddi v Sotheby’s [2018] EWHC 1491 (Comm).
  • Acting for the owners of a valuable 1932 Picasso (‘The Rescue’) in a dispute as to the existence of an oral contract purportedly granting the claimant an exclusive right of sale over the work with a 10% commission of the sale price (as sole counsel).
  • Acting for Sotheby’s in stakeholder proceedings over the lawful recipient of the net proceeds of an item that was sold at auction (as sole counsel).

Cleon has acted in a number of energy disputes, primarily in oil and gas related matters.

Recent notable work includes:

  • Advising a major international oil and gas company in respect of multi-jurisdictional claims for breach of loan agreements, breach of guarantees, restitution and fraud valued in excess of £130 million (with Hodge Malek QC).
  • Advising a consultant in the international oil and gas mining sector on potential claims in both litigation and arbitration against a major oil and gas company for the non-payment of contractual royalty fees (c. $3 million) in respect of the production income from a gas block in Africa (as sole counsel).
  • Acting for an engineering and manufacturing firm in the energy sector in respect of a c.£500k claim against a materials testing company in respect of failure to provide quality control services by way of non-destructive testing of welds (as sole counsel).

Cleon has a busy public law practice, encompassing judicial review, civil liberties and human rights, and healthcare and mental health.

He is the Editor (with Hodge Malek QC and Saima Hanif) of Atkin’s Administrative Court Practice.

Cleon has extensive experience in judicial review proceedings and has been involved in the following notable cases:

  • Acting for the Financial Ombudsman Service in a challenge to its finding that it did not have jurisdiction to review the manner in which an FCA review is conducted (as sole counsel): R (on the application of Mazarona Properties Ltd) v Financial Ombudsman Service [2017] EWHC 1135 (Admin).
  • Challenging the Secretary of State for Communities and Local Government’s 2017 business rates revaluation transitional scheme, which concerns the business rate liability of hundreds of thousands of businesses across the country for the period 2017-2022 (with Peter Village QC).
  • Acting for the Civil Aviation Authority in a challenge concerning its duty to consult in respect of alterations to air traffic control patterns to Gatwick Airport (with Gordon Nardell QC); and
  • Challenging the SFO’s refusal to permit legal representatives of choice at compulsory interviews pursuant to s.2(2) of the Criminal Justice Act 1987  (with Jonathan Laidlaw QC):  R. (Lord and Others) v Director of the Serious Fraud Office [2015] EWHC 865 (Admin).

Cleon’s broader public law experience includes:

  • Advising Jamaica’s Independent Commission of Investigations on the exercise of its statutory functions, including the compellability of information (with Hodge Malek QC);
  • Advising a major Public Corporation in respect of proposed re-classification by the Office for National Statistics (with James Strachan QC);
  • Advising County Councils on potential and actual legal challenges by service users and, among otherthings, the proper interpretation and application of the provisions of the Children Act 1989 and the Mental Health Act 1983;
  • Acting for a consortium of international media agencies in a challenge of the Enterprise and Regulatory Reform Bill which resulted in the Bill being amended (with Richard Spearman QC); and
  • Advising on the prospects of obtaining an injunction to restrict protestors.

Cleon also assisted Jonathan Cooper OBE of Doughty Street Chambers in delivering the week-long Advanced Human Rights Law Training Programme for senior Foreign Office policymakers and diplomats.

The Lawyer Awards 2022: Chambers of the Year