Waugh and Peace

I was thinking the other day about the funniest literary joke I've ever read.

It's a scene in an Evelyn Waugh novel but for the life of me I can't remember which one. It might be Brideshead Revisited. To paraphrase very loosely it goes something like this:

Sebastian, a dashing young blade, is sitting with his Uncle Basil, an old and very confirmed bachelor, in some ancient country retreat somewhere. Sebastian loves and admires his uncle but considers him very unworldly.

"Uncle Basil?"
"Yes?"
"Have you ever had a woman?"
Basil looks surprised. He thinks for a moment. "Yes," he says, a pleased, almost boastful expression spreading across his face.
It's Sebastian's turn to look surprised. "Really, uncle? When?"
Basil thinks for several seconds. "Once in 1913 and once in 1938."
"Good grief!" exclaims Sebastian, utterly amazed. "Who on earth were they?"
Basil hesitates. "It was the same woman," he says eventually, looking a little less pleased with himself.

If any of you out there have ever read any Waugh please put me out of my misery and tell me which book this scene appears in. Then I can rest in peace (Pretty poor when you've got to justify a punning title, but there you go, it is Friday after all).