“Spencer said the money had been advanced ‘relative to a political intrigue planned by Lord Castlereagh to abduct Bonaparte in 1803’, which was co-ordinated by Mr Liston, the British envoy to The Hague”

This directly involves Lord Castlereagh in a plot to remove Bonaparte from power. What is left for debate is whether abduction and assassination are the same thing. Clearly abducting the First Consul in Paris, unharmed, in the middle of his Guard and taking him to some Royal Navy ship hundreds of miles away is nearly impossible. So if Castlereagh didn’t mean to assassinate Bonaparte he must have considered and accepted the fact that he was very likely to be killed in an attempt to abduct him.