Hello all, thanks for all your replies. I tried BMW dealership, apparently they don't make keys for r100's anymore so the only thing they could suggest is to change all locks - costly! I called a couple of localish locksmiths but they were not interested. I'm not doubting that it can be done this way but as I live in the middle of nowhere, many many miles from civilisation, and I'm working 7 days a week for the next month to save my pennies for the big bike trip I've decided to go with the moto guzzi service. I called them and they were very helpful, for £35 I think it a good deal. I've sent the barrel off, I'll post back here to say how I get on. Tedmagum, it is a simple ignition but the moto guzzi service works out cheaper than replacing the barrel with a new one. New barrel is £67 from motorworks, no doubt I could pick up second hand but don't have the time. Also, this way I get my original shape key back, so I'll still have use of steering lock and be able to get under my seat otherwise I would need to change these too.

Thanks again

All is now well
 

Just to let anyone who finds themselves in my situation in the future know, Twiggers motorcycles were fantastic. The got a replacement key cut for me and sent back within a week. Very good service. If you have lost or broken your motorcycle keys, they are a nice cheap option to get you going again.

Hope you had two keys cut while you were at it. Will save loads of time.

As a follow up to a follow up it pains me to have to report that in the end I had to buy a new ignition switch from motorworks anyway. My old switch and new key that I got back from Twiggers was great for turning the bike on but after a short while I noticed that my lights were all screwed up. I couldn't have tail light and headlight on at the same time. The switch had been put back together wrong. Problem fixed with new switch.

At least I still have a key for under seat and steering lock. If you do use Twiggers just ask them to take care when reassembling.