my stepdad put about £20 petrol in our Peugeot diesel campervan last year noticed while at the pump, and brimed it with diesel. It drove home ok and seemed ok, but I didn't risk it and had it drained at our house. To be fair our camper van is worth a few quid, so worthwhile doing for us, cost us about £100 to drain as well as lost cost of full tank of fuel, so an expensive mistake.

As others have said, read that about 10% petrol is ok and in some cold countries they put a bit of petrol in anyway, but think we were borderline, so I didn't want risk it.