I've now found 4 or 5 threads all with the same problem, so a brief "howto"...
Got the switch off, not actually very difficult really:
1) Top & bottom covers off the steering column - three shallow screws underneath hold the bottom cover to the column, two deep screws underneath and two small screws at the front, behind the steering wheel, hold the top cover to the bottom. To get to the two small ones at the front involved a bit of shoving but got that done eventually.
2) Drill out the heads of the shear bolts holding the ignition switch on. Now, because of the lack of space, you have to go in at about 10 degrees off line so it's difficult to get the whole head off, but using a centre punch I was able to turn the remains quite easily.
3) Unclip the aerial unit (it's only on a clip ring around the front of the switch)
4) Unclip the two connectors from the ignition switch
5) Lift out the unit.
6) Once it's on the workbench, unclip the electrical connector from the switch to reveal the cog/operating wheel.
7) Take out the pin in the centre of the operating wheel. Don't drop it on the floor and spend 20 minutes with a magnet on a stick trying to find where the little b*st*rd went.
Aha! Pic 1 shows a slug of nylon that to me looks like it's knackered. Pic 2 shows where it sits in the operating cog.
What's happened is that the nylon slug has worn out. It's there so that once you've turned the key to "start" and let go, you can't accidentally turn it to "start" again with the engine running; you have to turn the key to "off" and then start it again; it engages with the sprung pawl you can see at the top.
I couldn't see a way of refurbishing or making a new nylon slug, so I just lifted the pawl out (Pic 3) - there's nothing keeping it in but make sure you've got the spring out as well - and you'll just have to remember not to turn the key to "start" with the engine running. Anyway, the grinding and crashing sound of starter teeth being knocked off will remind you.
Replaced the switch unit with a couple of 30mm long 8mm bolts and loctite. No problem.
Thanks to all for your help!