So I did this job over the weekend and, surprisingly, it was far less unpleasant than the Kohl/Widdecome scenario, apart from the fact that the new cylinder (with fixed insulation) was wider so wouldn't fit properly until I took the floor of the airing cupboard up and shifted a pipe. Much to my surprise, given the number of compression etc joints, the only leak was from the immersion heater seal - Not a crossed thread but it wouldn't tighten snugly against the cylinder. None of this was down to skill, simply to extravagant use of Fernox LSX, which also sealed the immersion heater - the best thing that's happened to plumbing in the past 10 years, IMHO. A close second was the ratchet pipe cutter from Wickes, which sorted all the cuts close to the wall...except for the run of stainless pipe, which was a PITA (dealt with using a jigsaw and a lot of swearing).
Thanks for all your tips.