The handbrake is a drum brake set up. I'd get the wheel off and try and back the shoes off. You can do this by the turning the disc till the hole is at 6 o'clock, take out the rubber grommet and you'll see a cog, turn that with a flat head screwdriver, can't remember if it's up or down. Once loosened off you will probably need to give the drum of the disc a few sacks with a rubber mallet to loosen the disc.
You should then be able to take the disc off and have a look.
It might need a new handbrake cable if it's the cable sticking.