I have always managed to get them loose with a long good quality ring spanner (and probably a little application of a hammer). I possibly used a socket with a long handle first time round, and that's just a case of jacking the car up to make a bit of space so you can get that essential initial movement. I was lucky enough to get new backplates just before they stopped making them and have tried to treat them and keep in them in reasonable condition because as has been said they are no longer available. I remove the rear callipers every year to have a go at the handbrake, its got to work for the MOT, and a good clean up is a good place to start, that and cleaning up the little pulley doodad in the middle of the handbrake cable which is perpetually seized.
