V8 Range Rover Petrol leak

My parents have V8 P38 which has been standing around as they now need a car with wheelchair access and its loosing petrol from the tank area but i had it checked over by a garage and they found no leaks and indeed after a run it had stopped but again its dripping have been stood for a couple of weeks (its on lpg so it doesn't normally use much petrol)
Its on ebay if anyones interested be quick 2 days left-its had much money spent on it in the past and it would be a good buy for someone with a bit of time.
I'd like to be able to tell the new owner what the problem is.
if anyone can help