Car: s1 RS turbo with cossie rear brake conversion

My passenger side handbrake is doing pretty much nothing and I'm baffled as to why. The piston in the caliper is not sized as I can turn it with a tool I made to wind it in. Also the rear brakes passed the mot, just failed on handbrake.

When I pull the handbrake on it moves the lever on the caliper but doesn't seem to move the piston.

Is there any special method to adjust it?