OK, to recap – when you shear off the high-tensile bolts in the ARB brackets, I found you have to take the brackets off to get the remains out. Not having a suitable drill at home, I used one of the ones at college 
The thing is seven foot high and has a chunk of torque in the low gears, so eventually the remains of the bolt were out. Cleaned the brackets up, repainted them then tapped out the threads in the captive nuts ready for refitting.
The new springs are from Rally Design and the difference in length is accounted for by a combination of the quoted two inch drop and their being a stiffer spring (145lb). OK, a firmer ride – but less wallowy and bouncy-bouncy, but if they don’t work out I can change them easy enough 
The struts and all their associated bits have all been cleaned up and given a fresh coat of paint, rubbers put onto the ends of the new springs, springs installed and everything bolted up again. I’d have liked to have gone to adjustable inserts while I was at it, but guess I’ll have to wait for Father Christmas on that one 
Existing dampers were checked for leaks, scoring, pitting and funny noises and had none, so will do for now 
Next job is putting the inner HD rubber bushes into the TCA’s … I’ve re-read all the suggestions, so I’m probably going to using a couple of sockets in a vice, all immersed in a cauldron of boiling water with lashings of KY jelly and handcream – just have to hope none of the neighbours stop by unexpectedly 