I failed to post a postscript to this episode.
I got a mobile fitter to come out with a pair of Primacy 4+, the replacement for the 4. Spacesaver back in the boot, fronts binned, reared rolled up to front, new on the rear. All went swimmingly until I noticed, ambling around the car while the fitter got the card reader set up, that one of the tyres was a Primacy 4+ and one a Primacy 4 (and therefore a few months older). Some swearing on the phone from the baffled fitter - I think someone at the depot had stickered them up wrongly, so someone else also had this slight mismatched pair - whereafter he had about 15 mins to source another 4+ from somewhere local, which he just about managed. Just glad I bothered to check.
And then the lesson in how not to do things: having equipped myself with a torque wrench and breaker bar as above, I went round the car undoing and then re-torquing all the nuts. Stood back and exhaled with pride at my manly competence. And then realised that I had done some tightening without having properly tightened the nut on the base of the torque wrench (so possibly damaged it). And then realised that doing it from memory I had done them all to 90 ft-lbs rather than 80. This is why I should leave it to the pros.
Anyway, thanks everyone for the advice: knowledge is power etc.
