......... Possibly the motor has died at
some time and someone has pushed the drive unit override lever across into
the manual position which sets the valve to pass hot boilerwater to both
cylinder and radiators. As the motor does not drive the valve all the way
across the internal limit switch does not operate to call the pump and
boiler into life. Turning up the water thermostat or running off some hot
water thus calling for heat to the cylinder does ask the boiler and pump
to operate and so you get heat to your rads.
............
Thanks for replying, I've spotted there's yet another control which I think
is the one you're refering to as it's on the valve.
It slides from "Auto" at one end to "Mid" at the other. It looks like you
can take a diversion on the way to Mid and go to an alternative position
instead. There's a picture that calls this "Manual Override Only".
The slider is set to "Auto" which I think you're saying is correct. (see
valveswitch picture at www.dulwich.net/ch)
Should it be set to one of the other settings?

Thanks