Siobhan Duncan

Siobhan practises across a broad range of direct and indirect tax work and is regularly instructed by solicitors, accountants, and other tax professionals in complex contentious and advisory matters.

She has a busy litigation practice, including proceedings before the First-tier Tribunal and Upper Tribunal, as well as advisory instructions. She has experience in appeals involving penalties, discovery assessments and capital gains tax, and is well versed in cases concerning failures to notify, deliberate behaviour and the interaction between substantive tax liability and penalty regimes.

In addition to litigation, she advises individuals, trustees, and corporates on complex tax issues, including matters with an international or cross border dimension.

Siobhan’s practice covers both contentious and advisory tax work, with particular strengths in:

• Capital gains tax
• Inheritance tax
• Penalties and compliance
• VAT

Tribunal and Litigation Experience

Regularly appearing as sole and junior counsel before the First-tier Tribunal, Upper Tribunal and Court of Appeal, her experience includes:

• Acting as sole counsel in First-tier Tribunal tax appeals, including cases heard in December 2025 and January 2026 and currently awaiting judgment
• Appearing unled before the Upper Tribunal in a tax appeal
• Appeals concerning penalties under the Taxes Management Act 1970 and Schedule 41 Finance Act 2008, including deliberate failure to notify and mitigation
• Advising on and challenging discovery assessments
• Transactions in securities appeals
• Capital gains tax appeals, including matters involving principal private residence relief
• Advising and acting in relation to accelerated payment notices and disputes involving the serial tax avoidance regime

Siobhan also acted in a pro bono Upper Tribunal appeal, Mark Campbell v HM Revenue and Customs [2023] UKUT 00265 (TCC), securing the first costs order in the Tax Tribunal in favour of the Access to Justice Foundation. This decision is regarded as a landmark in support of access to justice in tax litigation.

Advisory Work

Siobhan’s advisory work includes guidance for individuals, trustees, corporates, and professional advisers on:

• Capital gains tax, including disposals and transfers of residential, commercial, and agricultural property
• Principal private residence relief
• Inheritance tax and estate planning
• Capital reductions and corporate demergers
• Residence and domicile
• Stamp duty
• Stamp Duty Land Tax
• Transactions in securities
• Employee shareholder schemes
• Landfill tax
• VAT, including cross border electricity trading
• UK tax treatment of surrendered foreign life insurance policies

Drafting and Registrations

• Trust drafting
• Trust registrations
• Registrations of overseas entities

Education, Professional Memberships and Appointments

• LLM, New York University School of Law
• Bar Professional Training Course, City University London
• LLM, University College London
• LLB Law with French Law, First Class Honours, University of Westminster
• Certificat de Droit Français, Université Montesquieu Bordeaux IV
• Committee Member, Revenue Bar Association

Recent Cases

Mark Campbell v HM Revenue and Customs [2025] UKFTT 00867 (TC)

Osmond and another v Revenue and Customs Commissioners [2024] UKFTT 378 (TC)

Mark Campbell v HM Revenue and Customs [2023] UKUT 00265 (TCC)

Kenan Altunis and HMRC [2023] UKFTT 719 (TC)

Further First-tier Tribunal and Upper Tribunal matters are awaiting judgment or confidential.

Thought Leadership and Publications

An established tax author and commentator, Siobhan contributes regularly to LexisPSL and has been published by Bloomsbury Professional, Tolley Tax, Tolley AI+, LexisNexis and Croner-i.

Her writing addresses capital taxes, inheritance tax, intellectual property, and wider advisory issues, and is aimed at both contentious and non-contentious practitioners. She also produces practitioner focused commentary on developments in tribunal and higher court case law.

Siobhan is the author of a forthcoming book on inheritance tax with Bloomsbury Professional.

Other Experience

Before coming to the Bar, Siobhan worked in the commercial sector, including as a lead legal contracts negotiator at Imperial College London. She also worked at the Office of the New York State Attorney General in the Antitrust Bureau, assisting on high profile merger litigation.

Additional Information

Awards and Scholarships

• Pupil Award, Inner Temple
• Graduate Merit Scholarship, Alpha Kappa Alpha Educational Advancement Foundation
• Profumo Scholarship, Inner Temple
• Major Exhibition Award, Inner Temple
• Duke of Edinburgh Entrance Award, Inner Temple
• City Law School BPTC Award, City University London
• 21st Century Law School Scholarship, sponsored by LexisNexis UK

Languages

French, full professional proficiency

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