UPDATE!!!
I now have the tank back from the tank menders!! They pressure tested it and found 2 leaks, plus a slight driblble from the filler cap. They confirmed it's tin and therefore soldered the repairs. They also cut a further hole for access, cleaned out the inside of the tank completely, removed the filler type repairs that were there before, repaired the access hole, pressure tested again and painted the whole thing matt black. It looks pretty good to me and cost £440 in total.
I've refitted it, using new vent and fuel pipes and managed to relocate and tighten down the tank straps by myself, using slightly longer screws than were on before, so I could get the nut started. I've made 2 new neck filler grommet rubbers (Vitron sheet) which are now compressed between the tank hatch and the tank, when the filler cap is located, which SHOULD allow any fuel coming out of the cap vent to run over the wing, rather than into and under it as it may have done before, as there wasn't a proper neck seal there before. I've also made new cap seal rubbers also from Vitron sheet.
Putting the fuel gauge back in was easier than expected and I've replaced all the old tank hatch and wing fairing screws (1/3 of which I had to Dremel out anyway!!) with stainless steel ones sitting on nylon washers. I've ordered some heat activated v section rubber edging for the panels, which obviously protects the edges and should also help with sealing against fuel spills. I've used this extensively before on the Pitts and Andreasson and looks pretty good
I tested the tank for leaks with a gallon or so of fuel and lots of sloshing before fitting and a gallon or so after fitting and no leaks
I've run the engine and driven around on the ground for about 30 minutes and apart from a couple of slow wind downs in the first few minutes (I'm guessing air pockets) the engine's running fine
Would I have done anything differently, knowing what I know now? I'd have done the filler neck grommets and cap seal rubbers and re flown before sending the tank away, although because there were always some little bits of particles when checking the fuel drain (absolutely NOTHING in the fuel when checking post repair!!) I'd have still sent it away, even if the big leak had gone away
I'd also have refitted the tank hatch and checked the alignment of the neck filler against the filler cut-out BEFORE tightening down the tank straps, but that only cost me another 15 minutes of loosening, shuffling the tank and then retightening, so not the end of the world anyway
So, hopefully leak free now (I'll report back when test flown) AND I've learned more about the Cub and basically, removing the tanks isn't much of a big deal AND as I won't be having to cut out a bunch of rusted up screws next time, it'll be even easier
Any questions, please ask and thanks for all your support when I was in the dark days of trying to figure all this out
Cheers
Philly