charlie

i had the exact same problem on my estate, on the front nearside the piston gaitor/rubber had split on the caplier and was letting water into the piston which made it seize.

i could be on a small hill and it would hold with out the hand brake or anything from me!!

i also changed the caliper and found it was still doing the same thing but not as bad... checked the rest of the calipers and guess what, they were ALL seized or limited movement due to split gaiters.

in the end i changed all four and put in new pads and fresh fuild and problem solved.

might be worth checking the rest of calipers and hand brakes cable.