Hi Keith
I think we covered most of the possibilities but…
- the handbrake is mechanical so if it is not releasing will stop the caliper piston retracting sufficiently. As the pistons on both sides are affected I would suggest that if the handbrake is the problem, it is the first single cable to the lever that is too tight. I know that you have removed the handbrake covers and unwound but perhaps need unwinding a few more turns and pushing down.
- any proud corrosion on the piston under the gaiter will stop the pistons winding back past the main caliper seal. Wind the piston forward so the gaiter starts to extend. It can then be taken off the piston and pushed down the piston to reveal the state of the surface that has to get past the seal. Typically water can condense under this gaiter and cause rust. No real issue with pad wear and retracted position moving forward. Pitting will not cause issue but any proud rust will make it very hard, if not impossible to use the adjuster to get the piston in to the bore beyond that seal.If there is rust, wind piston forward, clean it all off flush with a stanley knife blade. Clear all rust debris out of gaiter and lubricate with silicone grease or red rubber grease, if that is what you have.
EBC Ultimax are a common pad for this application, so I think it very unlikely that these have been supplied with too thick a lining and not fit for purpose with newish discs. Most people fitting these would likely be fitting brand new disks anyway. Rear calipers are the same for all standard cars, MK1 to MK2.5, exception being SVT sport big brake calipers that have a larger piston.