Next week I might be able to spend some time working on the top surface. Which doesn't have any underseal or much rustproofing fluid on it and isn't nearly as pretty... I'm pretty sure that it'll be sound though, there are no pinholes in it. Yet. But it's possible that some wire brushing and paint removal may identify any areas of concern.. The welded flange around the middle of the tank is sound so I'm reasonably confident it's okay. The inside of the tank is showing surface corrosion unsurprisingly. There's a lot of dust coming out of it at the moment so that'll get hoovered out. Next test will be to block the inlet pipe and all the fuel lines and pressurise the tank to see if there's any evidence of leaks, find them and fix them before treating the inside of the tank with POR15 tank sealer to effectively put an internal surface in the tank onto a rust free surface. And then POR15 semi gloss black paint to the outside surfaces, fit the new sender and seal and put it on the shelf ready to be refitted.
Progress continues!