Thanks for the further advice, much appreciated. After another afternoon bleeding etc, and really getting no further (also tied the brake lever off overnight), I focussed in on your point about the over-sized o-ring. On the basis that the seal is not good, and air is working its way in past the o-ring. So, I have completely disassembled the caliper again. Below some photos of the calliper and the o-rings I have. When you say "oversized" o-ring, I assume you mean a 10mm but rather thicker? I cannot see how a larger o-ring would work, I have tried inserting the next size up and it just doesn't fit in the recess. If you have an e-mail or even telephone number I could give you a call to discuss. It's slightly driving me nuts, as I have been at this on and off for weeks!