thanks for advice guys, Dezzy I saw your repair thread, thats what I shouls have done, took it off the car and take apart. It wasnt the cable that was stuck, the piston itself was coroded to the wall of the caliper, no gap on one side, gap on the other side. WD40 didnt work. Unfortunately I lost my patience and went at it with a hammer
now its totally busted, cracked the aluminium housing.
I'm going to get a second hand one, not paying Audi £300 for a new one.
Lesson learned, dont let pads go down to the metal.