Hi guys - success!
Thanks for all of the good advice. I thought of using the heat/freeze option, but realised that my understanding wife might not be quite so understanding if things went awry...
In the end, a wooden table-leg through the spokes locked the engine nicely, and after that the bolt came undone. The rotor came off quite nicely - with a lot of leverage, a little swearing, and a judicious tap with a hammer.
Now - time to remove the belt, and there's a little phrase that has me scratching my head: "loosen the idle-roller nut and position the idle-roller in such a way as to enable the belt to come off the pulleys".
I've loosened it, but the roller doesn't seem to move much - a couple of millimetres perhaps. Certainly not enough to slacken the belt sufficiently to get it off easily.
Do I need to undo the nut completely, and remove the roller? There's a spring that keeps the idle-roller under tension - should that be removed as well?
Sorry about the never-ending questions...
A.