It certainly sounds like a seized brake cylinder Andy. All the symptoms are present. Worth pulling off the brake drum and having a good look. You might if you are lucky get away with a good soak in brake cleaner spray.
I'd also check that the handbrake cable is free from the adjuster bracket to the backplate as well. The cable outer can split at the bend to the backplate and the wire inner spiral corrodes and eventually snaps, wearing the inner cable.
Good luck!
