Johnmcmd your advice was spot on.
Made a jig out of 3/4 in ply, but found that cutting a snug fit was not a good idea with lots of dents as trying to get the damn thing over the tank was far too hard. (Also as I didnt know exactly how the things were constructed in the first place). I overcame the problem by cutting the plug 1/4 in oversize all around (still gave just enough material to drill fixing holes for the mounting brackets into the ply). I then made around 25 fingers that are either screwed down so as they can be easily removed to allow the tank to be removed or glued in place around the bottom of the tank...has worked a treat so far!
Deseamed my tanks today, took a bit of working out how they were constructed. Thought that the mounting plates were soldered on, and no amount of gental persuation and heat would move them (gas butane torch) so figured that it was either spot welded on or riverted, and didnt bother trying the other two. (and so they are!)
After a couple of hours of blasting away solder with my airgun and some gental levering under heat, the thing started to revel its secrets, and hey presto it was in 3 parts!
Metal condition better than I first thought, with only medium pitting to the petrol tank bottom only, no pin holes.
I am going to get the tank tinned before I reassemble them. I feel completely happy to solder the tanks back together again, its the tinning that will be a bit tricky dicky.
The one problem I do have is getting decent high wattage irons here in Blighty. There is a 300w model you can get, also I found a 500w model, but the tips look pretty small on the darn things!
I managed to find some really good industrial 500w irons with a good old size tip as used by roofers in the (Uwnited) states, but they are expensive with a capitol $$$!!
I am going to try some old solid copper soldering irons as used in the 30's. You know the type, you would heat them to operating temp then use the residual heat of the big lump of copper to solder with. I guess that this is what they may have used in the factory back in the 1920's/30's anyway? They certainly standard equipment duing the war for sheet metal soldering with the Army etc. I will try mine with my butane torch as it gives out around (I think?)1000 deg flame, about right.
Will let you know how the final result pans out, but so far it is very satisfying restoring something that plain looks UGLY!