Nice photos! It's a fairly conventional fully pumped Y-Plan system, but I
think you mis-described it a bit in an earlier post.
Sorry I confused things before - my ignorance knows no bounds on this
sort of thing. I thought the water was flowing in the opposite direction

Water comes from the boiler, through the pump, and up to the central inlet
of the 3-port valve. The LH outlet goes to the radiators. The RH outlet goes
to the cylinder coil. The bottom water pipe (not the gas pipe) comes back
from the coil and returns to the boiler. It is probably joined by the CH
return under the floorboards. The small pipe with the gate valve on it is a
by-pass so that if the heating is on but all the radiator TRVs are closed,
the water still has somewhere to go.

The fact that a part number is quoted for replacing the head almost
certainly means that the actuator can be detached. Maybe there are screws
going up into the bottom of it, through the brass flange on the top of the
wet bit of the valve?
Emboldened by your advice, I removed the cover from the actuator and
inside the casing I found two screws connecting it to the brass flange.
I removed them (carefully, as the thing was still electrically
connected) and tried to lift the actuator off of the brass bit. It
didn't want to come off - it wiggled about a bit but would not come
clear. Then there was a ping, like a spring jumping out of place and I
decided to put it back together and seek more advice. As far as I can
see, the spring is still in place.


Are you absolutely sure that the valve isn't returning to the HW position?
When it does, it requires pressure on the lever to move it elsewhere. when
it is in the mid or CH position, the lever flaps about without doing
anything. Note that in the manual position, the lever only moves the valve
to the mid position. There is no way of moving it manually to the CH
position.


I have checked again and the only situation where the lever moves of its
own accord is when I push the lever to the right (Manual), then put
Water ON while heating is OFF. This causes the lever to move back to
the centre position.

Something else that struck me as odd is that when I stopped fiddling
with this earlier this morning, I left the lever pushed over to the
left( Auto). When I went to test all this just now,some 7 hours later,
the lever was still on the left but the actuator was hot to the touch,
even though neither water or heating had been on for about 7 hours and
all of the surrounding pipes were stone cold. When I had the casing off
of the actuator, the heat was coming from the silver coloured drum
shaped object - presumably the motor.

Is this making any sense yet?