Mention of Lord Harrowby's house, No. 44, has deliberately been left until last. However, it is probably the most interesting mansion in Grosvenor Square because, from its connection with a well known incident, it hangs on to the skirts of history.
In 1820 it was numbered 39, the renumbering of the houses on the west and south side of the Square having taken place in
1888.
At this house, the Cato Street Conspirators planned to murder all the British cabinet ministers and Prime Minister Lord Liverpool, who were to dine there with Lord Harrowby — then Lord President of the Council — on 23rd February 1820.
As readers of the history of the period know well enough, this was a time of much popular discontent, and the moment was seized by one Arthur Thistlewood — who had previously suffered conviction for sending a challenge to Lord Sidmouth, and was a man "full of blood and fury" — to assemble a band of thugs as reckless as himself, and to formulate plans almost emulating those of Guy Fawkes – and, had they been successful, almost as far-reaching in their effects as the Gunpowder Plot.
The conspirators habitually met in a house in Gray's Inn Lane, and their deliberations seem to have been known to two Government spies, Oliver and Edwards.
At this time their plan was to assassinate the Ministers individually in their own houses - but finding that such a strategy would probably lay their scheme open to frustration before they had completed it, they changed their methods. After learning of the arrangement for the dinner at Lord Harrowby's, the venue was altered.
Their subsequent meeting, at which they put the finishing touches to their nefarious scheme, took place in Cato Street, Edgware Road, in the loft of a stable belonging to General Watson. It is said that, at one time, the larger of the two rooms into which the loft was divided accommodated no less than twenty—four conspirators.