So by way of an update to all the helpful people on this forum I have carried out a series of repairs/general maintenance over the last few weeks - Replaced water pump (Pierburg), gasket, LTT (Borg Warner), Coolant full flush and refill (G12), Belt and 3 Pulleys, aftermarket 993RS engine mounts, 6 Beru coil packs and Bosch plugs, Top Gear Sports silencers, Airbox resonator delete. All of this was fairly straightforward apart from changing packs & plugs 2 and 4, which was extremely fiddly with my socket sets and getting the old exhausts off which probably took the longest time. Everything went back on easily and the car fired up perfectly and with a very smooth sound. Whilst I did the airbox mod I cleaned out this disintegrating foam padding from inside the box which I would advise any 996 owner to check as you don't want that going into your intake, I also cleaned the throttle body (which was pretty clean anyway!) but didn't do anything to the MAF. Finally, I rust-treated and painted a few little bits of surface rust behind the bumper, cleaned the chassis and coated the underbody in Dynax-UC.
I have now had a chance to put c. 500 miles on the car now, mostly in wet/damp conditions with a broad mix of A-road/B-road driving with a few trips into town, etc. I have also been monitoring the car via the XTool ODBmini and app so I can get an idea of temps, etc. My impressions are that it is a wonderful handling car with brilliant steering feel (perfectly weighted, not over-assisted). The balance is pretty neutral, with a slight tendency to understeer which can be nicely controlled using the throttle to move the rear out mid-corner. That said, the levels of grip that the C2 has even on wet A-roads are such that you are going to be in three figures to get near the limit so really I've been mainly enjoying tight B-roads where you can sensibly get a feel for how the chassis reacts without losing your licence.
Compared with prior I think the biggest change has been the engine mounts, as the rear of the car feels really controlled and it is very difficult to upset the balance of the rear of the car, even over less than ideal surfaces. Where people always say cars go round corners 'like they are on rails' this doesn't feel like that at all. It feels more like water flowing round the corner, in that it moves lightly over the surface and seems to glide round the bends. I have yet to really get into the groove with the braking on this car – it certainly stops hard if you get on the anchors but I find it is quite easy to lock the fronts especially over rutted/stony surfaces. I have spent many years in cars with no ABS so I am fine with modulating the brakes in these circumstances, what does it normally take to trigger the ABS in these cars? No warning lights or anything like that so I presume the ABS is working, also car has PASM.
Next things on my list to do are to replace a cracked plastic wheelarch liner at the front and reverse the resonator delete! The car sounds fantastic from outside but it's a bit loud inside at 2000-2500rpm on partial throttle for my liking, so I'm going to try reversing this to start with.