Hot water puzzle.

What does the cold water do ?

AFAICT it is unaffected,as I run hot and cold for the bath...but that is in fact a damned good question.

Well done!

If it is unaffected that rather lets anything upstream of the tank out of the mix doesn't it?

I may try that now.

Fill the bath with cold water instead

There could well be a pressure limiting valve on the cold input to the hot cylinder - with the cold taps being fed from upstream of that valve.

The cold taps would then be getting full mains pressure, but the hot wouldn't. If that valve were faulty, it could restrict the hot flow considerably.

You really need to make some hot and cold pressure and flow measurements in order to give a few more clues as to what's happening.

Also worth noting I have encountered some taps that slowly reduce their flow rate of hot water as the body of the tap heats up - presumably expansion changing the internal geometry of the tap.

Any slim chance that some foreign body has got into the pipe work?

I have come across something like that where the object was drawn by the flow until it got to a constriction, in my a case a pipe joint and stems the flow. The user gives up and meanwhile the object drifts back by gravity down the pipe work especially if was at the top of a vertical run.

My situation was finding out why a pub lager tap stopped flowing and a small sliver of plastic from the disposable keg top that protects the outlet in transit had broken off and was small enough to get through the keg coupling.

GH

Yep, before I put a PRV in some of the hot taps here would do that when being feed by 4 to 5 bar. They'd go from a decent flow to a fast trickle once the hotwater (50 C+) started to come through. Traditional washer taps not 1/4 turn ceramic. These taps needed to be opened with care or the water would bouce out of the basin/sink. PRV set to 2.5 bar has more or less solved the problem.

Had a car which would simply cut out at odd times. Of course by the time we got to it, it restarted and went for another few weeks.

We had a lively debate as to whether it was a 1 in a million accident, or done with malicious intent.

Another interesting possibility

I am still checking out obvious mistakes..

The combinations of what happens at cold taps, other hot taps, before and after the cylinder PRV etc will probably identify the place to look first.

A good reason to vote Brexit ;-)