In the aftermath of the
Invasion of Isle de France in 1810,
Henry Brougham acted as defence barrister for some British prisoners who had joined the French. Harmer was their choice of solicitor. In 1812, following the
assassination of Spencer Perceval, the Prime Minister, in the House of Commons, Harmer was defending solicitor for the assailant,
John Bellingham, whom Harmer sought to defend on insanity grounds, but Bellingham was ultimately found guilty and hanged. In 1819 Harmer represented
Samuel Bamford after the
Peterloo massacre. Dennison, Harmer and
Charles Pearson were thanked by the "Report on the Metropolitan and Central Committee, Appointed for the Relief of the Manchester Sufferers" for their investigations.
William Hone's book ''The Whole Proceedings before the Coroner's Court at Oldham'', which exposed the use of force at Peterloo, relied heavily on Harmer's work with witness testimony. In 1820 Harmer took on the defence of
Cato Street conspirators. ==Later life==