Also, they are not repairable, so when removing them use a spanner on the flat sections of the valve body: the nut at the larger end of the valve should only be used to counter the force of removing the brake pipe union!. 

Thanks everyone. After re-bleeding the brakes several times, I managed to get what appeared to be even pressure on both rear wheels. I don't have a balance valve so that wasn't the problem, but worth a mention. A visit to the MOT testing station has confirmed that the brakes are now evenly balanced - and better than the previous visit - so the work was not in vain. I now have replacement wheel cylinders to hand ready for replacement in due course when the weather improves.....