Well, his mum took him to work and I had a play with his car. Offside rear wheel was totally solid and ABS light was on.

Brake fluid level was maybe a little high but not by much, so I stuck it in reverse and booted it. Nice black streak on the drive. Stuck it in 1st, closed my eyes (the garage is in front of me!) and booted it again. Darker black line.

Then I noticed the ABS light had gone and I was rolling backwards! No idea what caused it but it's working.

Could a drum brake freeze overnight even though it isn't frosty, or is something on it's way out? Just can't be arsed dismantling today in this wind.