Bodge up
A bodge-up but one that worked. Our sink tap is one that fixes via a large nut that screws up on the outside (I know there is a different sort of fixing). This one has tended to loosen up from time to time. The securing nut is difficult to get to with the spanner, so never entirely tightened.
The bodge was to clamp a bit of metal (an old drawer runner) to the outside with a jubilee clip and then fix the metal to the back of the sink unit. Solid as a rock now with no twisting around and it should stay that way.
I'm only mentioning this because I'm bursting with joy at having permanently overcome this most serious and annoying problem.
The only trouble was the awkward angle I had to work at. Gone and pulled a muscle in my leg.