My hot water tank has stopped getting properly hot. The heating is
working fine, so it's unlikely to be a boiler issue. The pump seems OK
and the thermostat ~seems~ to be operating fine (on the basis of a click
as you turn the knob). There's no air in the circuit judging by the
bleed valve. I'm tempted to replace the thermostat, just cos I can, but
I'm not convinced that's going to achieve anything.

Any (sensible) suggestions would be appreciated.

Standard open vented indirect tank and boiler with nothing fancy like
weather compensation etc. No recent issues. DHW timer set to run when
the heating is off.

Cheers people