As a results of what people said here, I appealed the 'failed' MOT and sent the bollox off to VOSA.
I have just got off the phone to them.
General synopsis:
1/. The car is now taxed and street legal and there is nothing they can, or will, do to affect that.
2/. If they find the car essentially roadworthy, they will take the matter up with the first garage. And refund my appeal fee.
3/. If they find it is not, they may take it up with the second, who passed the car.
This won't be till early January, but I will keep the groups here posted
followups (because albasani insists) to uk.d-i-y.