No prizes for guessing the location. It looks like at some point the inner wing has rusted out and somebody has patched it from behind. Now, the work appears - from the wheel side - very solid, it passed a recent mot etc... the door hangs well, and there is no evidence of any other issue in that general area.
Anyway, I thought that I could simply weld AROUND the patch, ie weld a thin fabricated patch all the way round between the inner panels and the inner-wing-patch. Would that be okay in the short term? As for safety, I thought of removing the petrol pump and feeds to the carb, then cover the petrol pipe with loads of wet towels, remove all stuff from inside glovebox, insulation etc, and wiper gubbings - then tack weld.
My welding would go here...:[frame]
The other side is slightly similar - I'd do the same here:[frame]