What is Clement Freud's £20 joke?

I heard a rather nice story about a man who drank a lot and his wife said, “If you ever come home drunk again, I’m going to leave you.”
He went out to a pub and drank a lot and threw up all over himself, and said to his friend, “If I come home like this my wife will leave me.”
His friend said, “I tell you what, go home and tell her somebody threw up over you and put a twenty-pound note in your inside jacket pocket, show it to her and tell her he gave you this for the dry-cleaning bill.”
He comes home and his wife sees him and is furious, but he jumps in, “No, no, no, somebody threw up over me and gave me twenty-pound for the cleaning bill.”
She said, “Well, why have you got two twenty-pound notes in your hand?” And he said, “Oh the other one is from the man who shat in my pants.”