"How much for full sex?" He asks.
"That be £20" she says.
"Ok" says the bloke and they get it on.
All of a sudden a policeman appears and shining a torch in their faces asks, "What's going on here then?"
"Nothing officer, I'm just having sex with the wife."
"Sorry sir, I didn't realise it was your wife"
"Neither did I" exclaims the bloke, "Until you shone that bloody torch in her face."