Strong smell of fuel

[Pb3]

On driving to Stoneleigh on Sunday I noticed a strong smell of petrol from inside the car. Strangely, it seemed to disappear once I switched tanks. Since I have been checking all over for any signs of a leak but can not see anything, although I can still smell petrol after the car had been standing for a while.

I guess only a tiny leak would be enough to give a strong smell, but how does such a thing develop on an ally tank? Is there any product that can be added to the tank to seal small leaks, such as RadWeld that is holding my radiator together!?