A very drunk fellow was staggering home via a very dark alley. He bumped into, er, a lady of the night.

Sez he to her, "How much do you charge for sex?"

"Twenty quid."

"OK!"

So they were getting on with it when a police officer came down the alley and shone his torch on them. "'Ello, 'ello, and what's going on 'ere then?"

"Er, I was just making love to my wife, officer..."

"Ah, I didn't realise she was your wife, sir..."

"Neither did I, officer, until you shone your torch on her face..."