Several more MoTs since last posting on this, the old girl keeps chugging along, not much to report apart from a few aircon problems, sorted with a new compressor.
However, recently when driving in this cold weather it's been really difficult to demist the screen, all the more surprising as I know the aircon is working now, after a mile or so the screen gets more misty than it was to start with, and even wiping it, it mists up again.
So, last weekend we did quite a long journey, long enough for the whole thing to heat up properly, screen was still misty but this time I could smell a faint smell of coolant.
So - heater matrix time. I got one from PFV, seems like a good item, comes with the foam strip and spare O-rings, I also ordered a new thermostat as mine seems to be taking longer than it should to get to the beloved "3 o-clock" position on the guage, and as I was draining the system I thought I'd do both at the same time. I've seen several stories on here of new thermostats going faulty quickly, so I opted for the more expensive genuine Volvo item - and it does look precisely the same as the one that came out, so happy with that.
The under-dash panels had been butchered by a previous owner, so just pulled down by hand - I'll repair them properly tomorrow so they fix as they should. Other than that, the process was pretty painless, took a couple of hours but (as usual) would be quicker next time after having gained the experience.
Best help was this video:
https://www.youtube.com/watch?v=_lKq7c5I5GU
Really well shot, well explained and was a HUGE help, make the whole thing very straightforward. He even tells you what size torx bits to use!
Bolts on the thermostat housing were VERY reluctant to come out, but using a technique of first hitting them downward with a club hammer (but not too hard!) then trying to tighten them up before undoing them, I managed it without chewing them up, and then the job was very easy. I cleaned up the old housing with a wire brush in a drill, put some copper grease on the cleaned bolts and mating surfaces, and it all went together again just fine.
Ran the car up and no leaks, lots of hot air from the heater etc. and no misting - job done!
Oddly, on the last long trip the car also put the engine management light on with a code for the cam sensor. The code disappeared when I reset it and hasn't come back, but as this is one of those sensors that will stop you dead if it goes completely I decided I should change it, so I've got one of those too and will change that tomorrow.
All in all a good day - will post back with how tomorrow goes, repairing the under-dash panels and fitting the cam sensor.
Cheers!