Decided to brave the snow today and pulled the pad carrier off. Outside pad was seized solid and had to be smacked out with the back of a socket wrench, and the inner pad wasn't much better.
Filed them both a little bit until they just moved freely, and refitted the metal bar thing and anti-rattle clips the correct way round.
Unfortunately due to combination of the fact the car is technically uninsured and the bad weather I was unable to take the car for a test drive, but I feel pretty confident the brakes will work properly next time.
Having said that, I was curious about something so I pulled the handbrake on 1 click and to my surprise the disc wouldn't spin. Pulled the handbrake on all the way and it held firm enough for me to torque the back wheel up!
The downside to this is that I almost certainly wasted my money on an unnecessary caliper, and a new handbrake cable. The upside is that my rear calipers are a matching pair now and I've got a spare handbrake cable and caliper in case something else goes wrong.
Oh, and my boot refused to lock when I'd finished. Classic.