I should probably update this. I've been edging closer to getting the car ready for MOT. I set about fixing the blowing (hastily repaired last summer) mid-pipe by attempting to fill a few pin-holes in the exhaust with weld. I'll not try that again, as I'm convinced I now have an exhaust made mostly of ground-down weld.

There is a time to admit defeat and buy a new pipe, and that was passed long, long before I finally got it to seal. The mid-to-backbox joint is blowing slightly, but that just needs some assembly paste. I hope the downpipe to mid-box joint remains sealed once the rear joint is sealed up, or I might throw my dummy out of the pram! In due course it probably wants a new exhaust, but this should do the trick for now.
I then turned my attention to MOT fail-worthy rust. The car is in a curious state...it has quite a bit of superficial rust which hasn't broken the paint but is worming around the rear arches and doors (so are a pretty low priority), but is pretty good underneath. Nonetheless, there are a few areas which needed some attention. I scraped off some failed underseal, cleaned the lightly corroded surface which lurked underneath with a paint/rust removal wheel in a drill, and treated the clean metal with Jenolite, zinc primer, and topcoat. I'll underseal it when I get around to it. I then started poking at the sills, and found some speed holes. Curiously the failure was with the 'middle' skin (castle section?), with the outer being ok, but you have to remove the outer to get at the middle skin. I have pictures ready for a rainy day...
I got about halfway through fixing one of the two (or maybe three?) holes in the NS sill (using a section of Swedish steel from a bent V70 door) when I started to run out of gas (for some reason, the regulator had crept up to 50psi, whereas I normally have it set at 7 or so), and the wind was blowing away what little remained, so the welds started to look crappy. I don't know exactly what happened, but somehow whilst getting frustrated with this, and trying to cut back the messy welds as quickly as I could, I managed to take the grinder to my finger, which stopped play for the day. It's only just healing properly now, 10 days later. I was very lucky I didn't do myself some proper damage there. A couple of friends who are doctors saw it and were amazed I didn't sever a tendon. Moral of the story? Respect the angle grinder, kids!
I had a bit of a poke about, and the other side needs a couple of bits doing too, plus there's a section of poor quality old repair which has failed and needs re-doing. There are a few more bits like that, which while still solid at the moment, will need re-doing in due course. Bearing in mind my dependence on the weather and the current rate of progress, I decided to switch attention to the 343 (which needs mechanical work, not welding), so the 345 is on hold for now. On the plus side, I managed to insure it for about £50 today, which was nice...
The car will be for sale in the next few months, so watch this space...!
cheers
James