Aye Ian,
Our council does MoT's, as does the local BT fleet workshop. I know someone, who is now an ex-council mechanic, as he could earn more offshore and he certainly had no fear or favours about doing the job.
It's just that you cannot be sure when purchasing a machine, what collusion may have taken place between the seller and the tester!

Rumbling main bearings are my look-out,

dodgy head bearings, you would have thought to have been picked up and failed upon.

I noticed the latter (having travelled 120 miles) and was then given a reduction in the price for the parts.

The old "Caveat emptor!"
My regards, Bill