As an update, it turns out I was barking up completely the wrong tree looking for a motorised valve. I'm now pretty sure my system doesn't use one - can't say if this is true for all Kingfisher 2's, but there really doesn't seem to be one at my property. What actually seemed to be the problem was that the pump had just stopped after months of summer inactivity. I was pricing up a new one but it turns out I was able to encourage mine to start pumping again with a few good whacks with a screwdriver :-)
Obviously I would never advise anyone to start hitting their delicate electrical equipment, but hey, it worked for me... When your heating's switched on, try feeling your pump: if it's vibrating, it's probably working, but if it feels completely dead, that's probably where the problem is. A new pump is around £70.
Saustin
October 2010