Oh well... I thought someone would have had some advice to offer, never mind.
In case anyone else has to replace their ignition switch this is what I did.
The two bolts holding the switch are snap off types and to make it even worse they are also glued in place. There is no way to unscrew them. They are absolutely bonded in place.
The aluminium casting that the switch bolts on to has to be removed by taking off the handlebars, unscrewing the big bolt at the top of the steering column and loosening the bolts at the top of the forks, it will now slide off easily and there are no problems associated with this. You will already have removed the headlamp and opened up the headlamp housing to reveal all the wiring bundled up inside so that you can pull apart the block connector and single pin connector which leads to the ignition switch, again easier than it sounds.
Now you should have the ignition switch in your hand still bolted to the aluminium casting and this makes it easy to take to the bench for drilling out.
The two bolts holding the switch in place have to be drilled out and as I do not have a suitably equipped mate to do it for me I bought a cheap bench drill (Sealey SDM30) so that I could drill straight and true and started with a 2.5mm hss drill bit drilled out the centre and then widened the hole to 3.5 then 4 and then 5mm at which point I was able to remove most of the old bolt and re-cut the thread using an M6 tap.
The most important part was making sure the drill hole was as central as possible, a cockup here would have meant drilling oversize and probably threading M8 or something. I did try at various stages using a left hand threaded bolt remover but the "studs" just would not budge, they had to be completely drilled out.
I bought a couple of stainless steel M6 bolts to screw the new switch to the aluminium casting and now just have to put it all back together again.
The bit that is going to stress me out now is finding a short circuit. The main fuse blows as soon as the ignition is switched on and I guess that in the accident wires got stretched and have torn somewhere, aaagghh..... At least mechanically it is sorted once more.
Incidentally I bought the new ignition switch on-line from Andy's Motorcycles and included a fuel filler cap and side panel lock so everything still works with just one key, less than £40 inc postage which I thought was pretty good. I have repaired the smashed front mudguard by lining it with fibreglass fibre and resin, it just needs sanding and painting. Now for the circuit tester.
Hopefully that may help give someone else the confidence to tackle the same job.