Found it. It occurred in 1999, I recall in was on the television news. I was in London at the time. He was driving on a motorway hard shoulder, he had his trousers undone. He said he had severe diarrhoea and needed to reach a toilet. The let him off, it didn't stick ! Advised to carry Bisto in the car.

WLD, not so sure it could happen to anyone. I keep my pants up while driving.