An observation, if I may:
Mini gearbox bolts are, from the factory, coated in a thread-locking compound that stops the bolts from unwinding in normal usage. If you're unsure, thread-lock is a thick liquid applied to the bolt thread before the bolt is tightened. After the bolt has been torqued, the compound hardens and acts as a sort of glue, stopping the bolt from unwinding.
If you have had a bolt come loose, my initial suspicion is the garage has not reapplied fresh thread-lock compound on reassembly. You could test for this by having an independent mechanic remove another gearbox bolt and check for residue on the bolt thread. It'll be a blue, red, or pink residue.
If you don't see it, or hardly any of it, I would suspect it hasn't been applied during reassembly, and this might provide you with an avenue to pursue in terms of demonstrating poor workmanship with the original repair.