just to tidy up my thread with a conclusion, it turns out that the problem was crossed hot/cold, caused by a knackered thermostatic shower valve. I didn't suspect it initially because the bath hot tap began producing only lukwarm water a long time before the shower valve properly failed (problematic hot water delivery was previously only intermittent, for the shower).

Basically, the shower valve was allowing flow between cold and hot inlets, even when turned off. This meant that when a hot tap was turned on, the flow from that tap was made up of boiler-heated water *and* cold water from the shower inlet, bleeding into the hot water pipes at the shower valve.

Regards.

Ant.