I took my leaky mower tank to an engineering works in town to ask if they could repair it. The bloke behind the counter backed away and said "Definitely not! That thing is a potential bomb."
So I took it to an old bloke who does welding and soldering in his back yard. He is a very good welder and I've got him to weld many things in the past. I showed him the tank and he backed away and said "No way I'm touching that thing!" He added "I will weld anything except fuel tanks." Then he told me how to weld it safely myself.
Clearly this is something you have to do yourself and "at your own risk". lol! The tank is still sitting in my shed.