Update.
I got a replacement caliper from Mark at Monash and fitted it last week. So far so good, the wheel is turning freely now or at least as freely as the other side. I think I will still rebuild this caliper and the LH one so at least I then know they are both good. I'll replace the pistons while I'm in there.
I've had a few attempts at removing the seized piston from the original caliper but have had no luck so far - it is well stuck. I've got it soaking in some penetrating fluid and will have another go over the weekend. I haven't tried compressed air yet, just a pair of vice grips.
