The saga continues... I took off the actuator head and took some pliers to the spindle on the valve to see if it would turn. It's just a little stiff, but definitely not stuck. I then did a quick test with the head still off to see if it managed to rotate when it had no resistance (i.e. without being attached to the valve), and it did nothing when the room stat called for heat - just remained stubbornly at 'W'. I thought to myself that maybe the motor had packed in. So I goes and gets a replacement motor and guess what... exactly the same thing happens. So that's ruddy annoying. What I did notice though is that there's a slight buzz from the motor whilst the room stat is calling for heat - I could hear it and also felt the vibration, so it seemed like it was trying to do something, but even with the motor freed from the assembly it still doesn't turn.
So again, it seemed like the motor is dodgy, but what are the odds of an unused replacement motor (genuine Drayton, sealed packet) being faulty in the same way? Made me wonder if there could be something wrong with the circuit board, although it looks simple enough, and there must be some power getting to the motor (the buzz/vibration stops as soon as the room stat turns off again).
Seeing as I still don't have a multimeter (for now, although I'm beginning to wonder if it might be worth getting one - and learning how to use it
) I've no idea what to do next really, short of stumping up for a whole new head unit and hoping for the best. If anyone has either a theory as to what's going on or experience of this issue then, as ever, I'd be grateful for any further tips (as you can tell, I'm reluctant to give in and call a heating engineer as it feels like something I should be able to sort out myself, but it may yet come to that...).