Well, what a mad couple of weeks with the car!
As I posted about already, it failed the MOT on the lower sills having huge holes, rear arches had holes, the rear nearside inner sill area round the back of the jacking point had a hole, was in a sorry state once I realised how bad it actually was!
After taking off the side skirts and removing the interior I also came across the offside outer/side sill having holes right through at the back (where these cars usually rust first) and the passenger seatbelt bolt was well and truly stuck and heavily corroded so in danger of breaking when trying to remove it, meanwhile a nice hole appeared a couple of inches away from it.
Got it in with somebody to get welded who has basically cut everything right back and started fresh, had the entire lower sills cut out and replaced both sides(!), a fair amount of the wheel arches replaced and replicated so it now looks original, inner and outer sill areas replaced too. Also patched the hole that appeared near the seatbelt bolt and repaired the area with the bolt itself.
He also found part way through that the corner of the passenger footwell behind the ECU needed attention, so a patch has been put in there, I also pointed out a broken tab on the front wing which is where one of the screws for skirts/mud flaps go through, it was previously snapped on the bottom so unusable, so that has been replicated too.
Quite a list of work! But I feel so confident now knowing that nothing else is likely to collapse due to rust in hidden areas underneath of the car. Quite literally added years to its life I'm hoping.
Had a retest for the MOT today and all was good except a rear brake pipe which decided to start leaking either just before or during the MOT... Got that sorted and replaced though and all passed now!
Added a couple of quick photos I took afterwards but I'll try and take some better pictures soon, it literally hasn't stopped raining since I picked the car up from welding on Saturday afternoon, thankfully it was all undercoated there and waxoyl sprayed inside the new sills. I just need to get the side skirts back on and see how well the new holes line up for the plastic poppers underneath.