It was therefore fitting for it to be the venue for a mock treason trial of the Barons who negotiated the sealing of Magna Carta in 1215. A sense of pageantry was instilled by a procession of Serjeants at Arms from the Commonwealth and the USA, whose ceremonial dress brought a Gilbert and Sullivan feel to the event.

Some 800 members of the public had secured tickets to observe James Eadie QC, for the prosecution, and Nathalie Lieven QC, for the accused, argue their respective cases. Four witnesses were called, each played by a modern day counterpart. Former Lord Chief Justice, Lord Judge, inhabited the role of William Marshal, Earl of Pembroke, as if born to it, bringing to life the stone effigy in the Temple Church. Barrister turned comedian, Clive Anderson, gave a portrayal of King John which unleashed an element of camp, previously unremarked upon by historians. Lord Lisvane, not long retired as Clerk of the House of Commons, was a magnificent Stephen Langton, mixing piety and haughty authority as Archbishop of Canterbury in a way that Justin Welby can only dream of. Historian Professor David Carpenter, who as the nominal defendant Baron Robert FitzWalter stood to lose his liberty and his life in the event of a conviction, seemed surprisingly the most relaxed. He bantered with the court, even teasing prosecution counsel for a slip of the tongue in referring to the signing, rather than the sealing, of the Charter.

The scenario was entirely fictitious. Notwithstanding the troubled times, no treason trial arose from the events of Runnymede. But every effort was made to create an atmosphere of historical accuracy with an illustrated souvenir brochure, and narration by professional broadcaster, Gavin Esler, in the manner of a Pathé newsreel.

There was humour too – with varying degrees of subtlety – including contemporary allusions by counsel to withholding evidence due to national security, or for the protection of sources, and to the threat of governance from Europe. King John, very much in the style of a pantomime villain, reminded the bench (more than once) that they were his judges.

Baron FitzWalter’s appeal to clauses 39 and 40 of Magna Carta and his application for trial by his peers was summarily rejected by the court: in part because the Charter was so new its provisions were still uncertain, but mainly because of the distance travelled by the distinguished panel of judges – from America, Stephen Breyer, Associate Justice of the US Supreme Court; from New Zealand, Dame Sian Elias, the country’s Chief Justice and (less convincingly) from the other side of Parliament Square in the case of Lord Neuberger, President of our own Supreme Court.

After retiring for a respectful period, the bench returned a unanimous Not Guilty verdict. Justice Breyer departed from the conceit of the evening and delivered an impassioned encomium of Magna Carta, or at least its spirit as now embedded in US constitutional law. Dame Sian analysed the evidence, clearly finding William Marshal a credible and persuasive witness. She rejected the contention that the Charter was extracted from King John at swordspoint. Lord Neuberger, concurring with Dame Sian, concluded that the prosecution had failed to prove an essential ingredient of the common law offence of treason, namely that the acts of the defendant barons were “unjustified”. On that narrow issue, the anxious looking barons were acquitted.

As treason trials go, it had proved a jolly good night out.

A video of the event, which was held on 31 July, is available to view at www.youtube.com/uksupremecourt.

Contributor Mark Hill QC, Francis Taylor Building 
Mark is co-editor, with Robin Griffith-Jones, of a volume of essays, Magna Carta, Religion and the Rule of Law (Cambridge University Press, 2015).