Ok Martin, we're there towards the end of Jan. Please be aware that not every ones knee problem and knee physiology is the same and an inappropriate or ill-fitted knee brace could cause further problems. (My wife's said I must mention this due to her duty of care as a qualified health professional)
Lets hope the snow builds up nicely for your trip!
I recently went to a specialist in London who recomended the following,
Try the 66fit elite stabilised hinged knee brace for skiing. http://www.amazon.co.uk/66fit-Stabilized-Hinged-Patella-Support/dp/B005JBP974
I've used it and its very comfortable and stops the knee from twisting.