The piston is sticking inside the caliper itself due to the seal being cracked.
My main concern with ordering off their site is due to their eBay store being closed and no recent threads regarding them.
How long ago did you get these Rich?
about 3000 miles/ 6 month ago now,
I bought a set of calipers to refurb for a fair price on the sale section and swapped them over with mine once i'd done, I stuck new pistons and seals on them both. I'm doing my old set at the minute, just waiting till payday to get another seal kit!
Delivery was top notch, I'd try get your hands on some stainless bleed nipples too (might as well do the lot cause they will need a bleed), really worth the few extra quids! I swapped all mine from mild steel no more siezed nipps