On Sun, 18 Oct 2020 09:09:04 +0100, Graeme wrote:

In message , Graeme
writes

I'll need to dismantle the valve a little, just to see whether it is a
broken spring, but what I'm not sure is how much dismantling can be done
without water coming out unexpectedly!

Doh! Just looked on YouTube and answered by own question. I can see
that releasing two screws enables the whole motor unit without releasing
water.

Unless it is 20 years+ old, which does release water if you take the head
off. If it has a flat top cover, then it is old, and will probably need a
partial drain off, and complete new valve.