Its leaking from when the filler neck goes into the tank. Once panel beater said if its to be soldered, its not a big job. If its to be welded, it is a big job. Did anyone had this done before? Its not a VW tank
Raf

had a tank welded recently. friend did it at home with an oxy.
to stop it from going bang, he filled the tank up with carbon monoxide.
ran a hose from car exhaust into filler neck of tank.
CO gas doesn't combust, and is denser than air, so wont just drift out the neck, rather displace petrol vapor and fill the tank. making it safer to weld.

*note use of word safer, not safe.

Sh!t, that's scary