The chances are you will need to see a gearbox specialist. Even if a garage did offer to take up the job, personally I would rather see a gearbox specialist. Gearboxes are fiddly things and I wouldn't want someone to do it wrong, and it's not easy to remove/fit a gearbox! While your getting the LSD fitted, as Sparky mentioned you may as well get the diff bearings with seals and gaskets, but I'd also replace the syncros for peace of mine. I did this with my old Glanza and the gearbox was a beauty. It cost £150 in labour from CogTeg Transmissions in Bradford, who I would highly recommend.