Short answers, pulled from the story.
Robert Jenkinson received a direct invitation from the Prince Regent on the 8th of June 1812. This event occurred just days after Spencer Perceval became the only British Prime Minister ever assassinated in office.
Spencer Perceval was the only British Prime Minister ever assassinated in office. John Bellingham shot him inside the House of Commons on the 11th of May 1812.
The tenure lasted fifteen years starting from his appointment in 1812. It continued until April 1827 when Lord Chancellor John Scott maintained his position continuously through that date.
Viscount Castlereagh died by suicide in September 1822. His death forced a rapid reshuffle where George Canning assumed the Foreign Secretary role.
The Peterloo Massacre took place in August 1819 and tested government authority. This event happened during the transition from war to peace after Napoleon's defeat at Waterloo.