Hi Paul.
Sorry to hear that.....
As ever, my starting point is Kryten, the repair guy based at Aperture's shop on Rathbone Place, just off the north side of Oxford Street. I can't guarantee he'll be able to fix it but I have yet to present him with anything mechanical that he couldn't make work.
Aside from him, I have a friend (a professional photographer and proprietor of a camera shop) based in the next town along from where I live and who has access to a chap who has also done a really good job fixing stuff for me - but he tends to take more time about it. Please let me know if you want me to have a word with him.
Paul.