i had a similar problem with my car... turns out the rubber gasket at the top of the tank was not fitted correctly when the previous owner changed the fuel pump. it would pay to check the lid and gasket if yours was changed...

on mine, it was pretty clear that's where the leak was, as the petrol was peeling off paint etc on the outside of the tank where it had been slowly running down. inspect the tank and you should see where it is coming from....