Surely JennieG you already know the tune to "Let's do it" as it's a parody of the old standard of the same name. [LET'S DO IT (LET'S FALL IN LOVE)] Didn't Fred Astaire and Ginger Rogers sing it?
My vote goes to "THE WIDOW'S PROMISE" too as mentioned by Cavia P. Mick Ryan always introduces this song by saying he was reading an article in Cosmopolitan in bed one night on women's sexuality. The fact that they have more sexual stamina than men gave him the idea of this marathon between the devil and a widow who lived on the moor. She wins of course. The song is in the DT