Right I went ahead and pulled the caliper apart tonight. From what I can tell at maximum handbrake position something slips a bit, like its gone past its stop position, and let the piston return slightly. I guess this is why it's not clamping properly.???

So from what I can see it must be either the socket on the bottom of the central screw, the little bullet or the socket on the handbrake rod? Either 3 could be worn??

As you can see from the pics the caliper isn't that dirty and looks pretty new. I guess they could have been reconditioned recently? Can these bits been bought separately?

What do you guys think?