Cartwright's sitting room in Cornwall is actually Rolf Engstromer's entrance hall at Eltham Palace. The same room (and indeed the exterior) stood in for Wode Hall when the same production company made their version of "Death On The Nile" in 2004.
The dinner party given by Bartholomew Strange is in honor of the St Leger horse race. This race also figures prominently in "The A, B, C Murders".
There is a slight in-joke / reference to the previous episode 'Appointment With Death': when Sir Charles Cartwright (Martin Shaw) is describing his dramatis personae for the cocktail part, 'Egg' Lytton Gore is seen reading a book called 'Travels in Arabia' by Dame Celia Westholme. Dame Celia Westholme was a travel writer in 'Appointment With Death'; coincidentally, in that episode the character was played by Elizabeth McGovern, who appeared as Lady Marguerite Blakeney in the BBC's Scarlet Pimpernel series alongside Martin Shaw playing Chauvelin
Martin Shaw (Sir Charles Cartwright), Ronan Vibert (Captain Dacres) and Suzanne Bertish (Miss Milray) all appeared in the BBC's The Scarlet Pimpernel (1999 - 2000)