| Hi Tonio,
There should be a few around for £50-70, but remember it will need servicing, re-springing and setting up before you get any benefit from buying it.
I paid £90 for my NCR shock and it's miles better than either the Sachs or the Showa, so my advice would be to hold out for a decent shock off fleabay.
I wouldn't suggest trying to get it sorted for Cadwell. Once you get it it'll need setting up which will take a few track sessions. Not a good idea unless you've got a track day before the race.
Cheers,
Ali | 
Threaded Mode