OK I fixed it :)
For the benefit of anyone else with this question, I read on
this yahoo page that corrosion in drum brakes may cause them to lock. As was suggested, I tapped the drums with my wheel brace but this didn't unlock the wheels.
So I loosened the nuts, jacked up the left rear wheel and heard a clicking sound from the brake. After that the wheel was free. I did it again on the other side, only this time I had to muck with the nuts a bit more but that released it. I haven't had a proper drive yet, but I couldn't hear/feel any scraping and the handbrake still works (hence cable isn't broken) so I assume the car is safe to drive, at least as far as a garage
:)