If I am right then the likely cause is people attempting to engage a gear from neutral with the engine running. Which the cars handbook expressly tells you not to do.
If you are right and this is what has happened then do you expect every garage to read every single cars handbook to check how to put a car into gear?
If the car is this specific and the gear selection arrangement so critical (not to mention slightly strange) then surely you would have told them this when dropping the car off rather than assuming they'd read the manual for what is something so normal on every other car I know?
I'll be completely honest Paul, a lot of this story doesn't quite add up and it seems you're trying to point the finger of blame when you're not even sure yourself what's gone wrong. Get the facts clear first before assuming it was some kind of negligence on the garages side of things.
For example, the carb...you say it was 'gumming up' and you knew about this yet you still took it for the MOT. Then let the garage take the car apart because you didn't trust them enough to take any tools with you and do it yourself. Surely this is something you should have sorted before the MOT? If it really is a case of six screws to get the jets out then have you checked that the six screws are back in place (I'd imagine this would be a fairly simple check based on your comment that this was a simple job?) Other than that, what else could they have dropped into the engine?
As Soopahfly says, what kind of a rapport have you got if things are known to have gone missing in the past? Yet you've "literally had him do hundreds of MOTs for me in the past"
It seems a lot of speculation and accusation without any kind of evidence to even begin to blame anyone. I know you feel frustrated, I would too, but check and find out what the issue is first and foremost before specualting and accusing. Especially if the garage is that good that you've had hundreds of MOT's there....with that in mind, you're obviously a VERY good customer so they would/should be bending over backwards to help. I can't imagine many people in your area taking this many cars to them for MOT's so if the rapport is that good then surely they can't be that bad or negligent?
Asking if you have a 'case' against the garage is utterly pointless when you don't even know what's wrong. Like I say, things don't fully add up right now, so get the car back, find out what's wrong and take it from there. Cars do break....that's why mechanics exist and there is always a slim chance that they might just break when the car is in their hands. After all, isn't this a car that's been off the road for many years? You've only had it running very recently and your first test drive it broke down. Cars that haven't been used for a long, long time tend to have a habit of doing this so there is every chance that it could be something compeletely innocent.
Establish cause then decide if there is grounds for negligence before making posts like this. In the meantime, find another MOT station if you're stuff tends to go missing from the car. Heaven forbid you left SWMBO in the car on a test!!