check your cables are not sticking,and also the caliper handbrake lever arms arent seized,then do the following.

.

.

.bear in mind if you move the caliper levers at any point during this proceedure it will reset the caliper and you will have no handbrake as you do now.

.

.

remove plastic undertrays
slacken off handbrake cables completely
undo lower caliper bolt and swing caliper up
remove pads

grind the little nobble off the inside pad backing

(it makes adjustment easier and serves no real purpose-its there to stop the piston unwinding which is nigh on impossible,remove it and life is ten times easier)
turn the piston clockwise until it will go in no further,you will be able to turn it but it wont actually go in anymore
fit pads copper slipped up
wind piston anticlockwise 180* and try to slide it back over the pads,if it wont go then take the piston all the way back in but add 10*-20* of clockwise rotation,then back out 180*.

.

.

.

if its a slack fit keep turning the piston anticlock until you have a good interference fit between caliper and pads

(this is where removing the metal nobble from the inside pad helps)
if at any time you accidentally moved the lever arm start again
when both calipers are bolted back in place with good fitting pump the brake pedal several times
take up the slack on the handbrake cable so you have 3 or 4 clicks
go and make a cup of tea