On 2020-05-01 23:21:24 +0000, Davey said:

Was this ever resolved?

Well I've finally got around to getting under the stairs to investigate.

One thing for certain - the motorised valve controlling the hot water
was jammed half open, and the valve body has a very small leak but it
seems to have destroyed the motor body, which is a shame as I only
replaced both actuator heads recently (the other for the heating
circuit).

So I need to replace the valve body but that's beyond me although I do
have the part as I bought it with the motor actuator. But It looks like
I need a new actuator...

i presume the system needs a drain down to do the valve but it's just a
couple of large screw nuts.

At the same time I guess I should get the expansion vessel serviced. I
think the boiler is fine - it's a bullet proof Valliant that was
serviced a year ago.

I've freed the valve spindle and applied a bit of lubrication for now
and will operate it manually til I find someone who can replace it and
the motor head - unless anyone thinks I can do it?