Thanks to solemnsleep for the link!
Drawing of William Pitt by Henry Edridge, for sale at £180. This is what Stanhope says about it:
… the drawing by Edridge taken in 1801, and engraved in the course of the same year. It is a small full length, which represents Mr. Pitt in his every day attire and seated at his usual writing table. I have heard several persons who were intimate with Mr. Pitt declare themselves much struck at the faithful resemblance of this print.Lord Stanhope, Life of Pitt, vol 4
The garment draped over the table would be the ceremonial robe of the Chancellor of the Exchequer. I hope he didn’t really just leave it lying casually around the office like that! Though I don’t actually know when he would have had occasion to wear it - maybe on state occasions like the opening of Parliament?
I’d love to think the view behind him was real - St. James’s Park, perhaps? But the “window” is all wrong for Downing Street, and in fact it probably isn’t meant to be seen as a window at all, but rather as an idealised backdrop. The actual view from the back garden of 10 Downing Street, painted in the 1730s, can be seen here.