Scott has brought this to us :
That sounds more like a faulty diversion valve.

Is this what used to be called the 'motorised valve' when I was a boy?

Yes! You should be able to put a hand on the two outlet pipes and if
there is only call for one - HW or heating, only one should be hot.


Thought so, my neighbour's is even worse.

Our boiler is in a cupboard on the wall and is almost inaudible. I have
to open the door to hear if it is running.


That's one for the OP. My washing machine is cold water (LG) for
which I am grateful given the unpredictable behaviour of the boiler.

Quite easy to get a Y connector and feed cold to both hot and cold
inputs.



As a complete diversion, if I wanted to replace my boiler with a mains
water filled storage tank is there a market for second-hand boilers or
would economic and regulatory issues consign it direct to the scrap
heap?

There is no market for second hand boilers, much of the cost for most
people is the cost of installation. The might have some slight value to
a DIY installer.