The gear box was slipping in third, and has recently started to jolt when changing up. l am hoping even after the garage has made it worse that it is just overloaded with oil and that a complete gearbox service will fix it, l have found a specialist garage to do this. Is the gearbox fixable and if not how much is reasonable if l have to pay for a new one. Some are quoting £300 for a reconditioned one that may be just as bad as my own after 90 days. Your advice is most welcome

Have them correct the level first to see if that fixes the problem. If you're lucky, the problems will disappear.