Just had gearbox repairs and now it jumps out of reverse.

Had slight whine and my local mechanic wasn't sure if it was gearbox or thrust bearing and he suggested a specialist. They checked it and said gearbox and changed main input shaft bearing. I asked to have full clutch assembly and flywheel done due to labour costs and having bought a caravan and wanted it tip top for towing. First time I went to fetched it they couldn't get it to run properly and had fitted the wrong flywheel. Next time I fetched it I drove home great and when I to reverse it to go out later it made a terrible crunching noise and jumped out of reverse. Went back again and they said bolts holding reverse bearing weren't tight enough. Went to fetch it again today as they said it was ready and its done the same again, so I've left it there. I've already paid£1000 for this work and I don't know what is wrong. They have had it in their workshop for over 5 weeks when it was originally going in on monday and ready for friday, joke. Could they have done damage driving with wrong flywheel? Before he knew it was wrong flywheel they couldn't get it to run properly and they disconnected a small circuit box and it ran better but they said it shouldn't have. I don't understand any of that but thought maybe it would mean something to you. I really hope youv'e got some ideas as I've now left my vehicle there again and don't know what to think and am very worried whats wrong and where I stand as he's already had his money.