the way to do it is find a big pub on a main road with a massive car park,rent a section of a car park and get loads of cash in hand workers,sit down and count the money!
mobile valeting is a waste of time imo,i done it a few years back,if you get in with garages you can make a bit and also buy cheap pxs and sell for a decent profit,apart from garages and companys nobody likes to pay much as they would rather pay £5 at these places and supermarkets