How to change the rear pads on a 05 Peugeot 307 XSI?

Have got access to the calipers and removed them, but then find it impossible to release the pads to change them. Normally you can push the piston back and relaese the shoes but not on this caliper. What is the secret.
I suspect that you may have to release some kind of self adjusting mechanism, but how and what special tools are required?