Hi Fraser, I have the same model and did have the same problem of showing the gearbox fault and going into limp mode. I only had the car for a couple of weeks so took it back to garage where I bought it from and they tried all sorts of cures including replacing EGR valve. It was ok for a few days but then started with the problem again. After agreeing with the garage that they would be paying for repairs I took it to Jagutech in Hull and they replaced the turbo actuator, although they did say that there was quite a lot of carbon in the turbo vanes. I dont know how much everything cost as the garage I bought it from paid for it. It has been ok since and that was 50k ago !! I`m fairly sure that you can get a recon turbo for about £500 but dont know how easy it is to swap them over. Hope this helps.
Cheers
Rod