My central heating system has an electric valve in it, which controls
whether the hot water produced by the boiler goes to the radiators, to
the tank to heat water, or to both.

A few years ago I had to replace the valve because it was sticking and
now it's doing the same thing again. I noticed at B and Q that you
can replace just the motor within the valve rather than the whole lot,
so I did that. It seemed to sort it at first but after a few hours it
became clear the problem is still there.

I'm wondering if the valve contains some sort or circuitry that
controls how the thing operates - I guess it must because it seems
able to tell the valve motor to operate - and I wonder if it could be
that that circuitry has gone, in which I will need a whole new valve?

It seems to me that the valve itself is working in that I can move it
manually using the lever, and I live in a soft water area so it can't
be blocked with hard water deposits.

What happens is that the valve tends to stay in the position for hot
water only, even when the hall thermostat is calling for heat - I can
push it into the middle position with the lever. In the morning when
everything is cold, it comes on and heats the water but leaves the
radiators cold.

Any ideas please?!