All done, but embarrassingly, not by me.
I got all my tools out, laid them down by the car, took the wheel off and could I Hell remove the disc securing screws. I tried the impact driver but they refused to budge. Then I attacked them with a punch, but still couldn't shift them at all. Worse still, I left the heads of the screws in a pretty sorry state after bashing away at them for 15 minutes. And after all this build up I was SO looking forward to doing the job.
Anyhow, my next move was to call out a mobile mechanic whom I've used for years and ask him to have a crack. He removed the wheel, put the business end of a Philips screwdriver in the head of the disc screw, hit it a few times with a hammer and lo and behold, out it came, along with its neighbour. Same result on the other wheel. Red faced, I walked away and left him to it.
Cost me a few quid, but the main casualty was my pride. Still, if I keep the car long enough I know the job will be a cinch next time round.