Hi,
I had a similar problem. It turned out that the seal on top of the fuel tank was leaking, but only when you the tank was filled to the full mark. A small amount would leak over the top of the tank down the sides and then directly underneath the car.
Remove the inspection plate located in the boot. This will reveal the access to the top of the tank and fuel pump. Go to your local servo and fill up your tank till it's full. Keep an eye out on the top of the tank and if you notice fuel pouring out of the top (trust me you will notice it) then its a good chance that it's the seal. It would also be a good idea to check all the fuel hoses going to and from the tank and check for wear or cracks.
Hope this helps
Regards