Thanks Marlen. Is panel bond adhesive similar to JB Weld? Definitely agree about the welding, I've viewed hours of youtube videos now and have decided on a gas MIG. I was planning on using my dremmel to cut out a small rectangular hole in the tank, then cut the same gauge steel sheet slightly larger, but overlap on the surface but thinking about it I could, like you say, slide this inside, hold with magnets and weld inplace. I will look into the panel bond adhesive too but I think I'll weld it mainly for a perfect seal and also because I don't want to risk any 'gassing' coming off and wrecking the finished paintwork.
Paul
Paul