Hi Iain, yes fuel tanks can be repaired if you have the skills, tool, patience and time. generally this involves cutting out the damaged areas, panel beating new steel to the required shape and size then welding them in place.
The biggest problem with this is that fuel tanks usually rust from the inside out, so you will still have rust inside the tank which will contaminate the fuel. There are rust treatment sealers for the inside of fuel tanks, they are quite expensive but they do work.
Let us know how you get on.
Bob