"and champagne cocktails at Harry's bar"
Harry's Bar is a Venetian bar and restaurant. It was a famous haunt of the writer Ernest Hemingway. As it did not open until 1931, Waugh has made what Lord Marchmain would refer to as a bêtise.
The Bellini, a cocktail made from prosecco (a sparkling wine) and peach purée, is said to have originated there.
The original Bellini recipe