On 08/11/2017 06:42, wrote:
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.

Thanks for the update... one to keep in mind for future reference!

--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/