Reference is made to two dubious car dealers called "Dudley" and "Dunstan", described as "the winsome Welshmen". These are the two dodgy dealers in the 1960 film, played by Dennis Price and Peter Jones.
Morse tells Joan Thursday she resembles "the girl" in this film, meaning the Rita Tushingham character. He also quotes the last line of David Lean's film, "Then it's a gift!".