So here's an update I wasn't expecting. The garage just called to say there has been a development - there's nothing wrong with the car at all!
They claim that having put it on a ramp (not sure why it has taken 3 weeks to do so) they immediately saw a shaft wasn't turning, traced it back to a ring and clip that had not been reconnected properly after spring arms on the axle (I'm not a mechanic so I'm going off what he who is also not a mechanic told me) had been replaced which required the shaft to be temporarily disconnected to do so.
They have replaced the clip and claim that the car is now working absolutely fine. The trader now wishes to give it back to me.
I was a bit taken aback on the phone so I said I would call him back on Monday. But thinking about it, a week ago they told me something had sheared in the gearbox. Not that it was possible something had - but that something had. He mentioned just now that they had been looking for replacement gearboxes but there weren't any in the country as apparently they never go wrong.
I have to question just how much of a gearbox specialist the garage it was sent to is as if this was actually the problem, I could have had the car back weeks ago. Surely one would trace backwards to where the problem could originate before deciding that the gearbox has definitely failed.
My instinct is that I don't believe them. But given they have repaired the car do my rights even apply anymore in terms of a refund? I am always going to be suspect of whether they have told me the truth and it will therefore be in the back of my mind every time I accelerate that the car could fail at any moment so at this moment I am in the mindset of not wanting it back.