The radiators have stopped getting hot but the water is still doing so.
Boiler fires but when set to heating only, quickly stops again.
Zone valve is the Honeywell V4073A1039 which is in the airing cupboard.
I've read through <http://www.diyfaq.org.uk/plumbing/controls/midpositionvalve.htm#Heating which I think describes this type of valve.
It seemed to me that the valve control might have failed but having taken the cover off I can see movement when the CH timer is set to water. However, when the CH control is set to provide heating nothing moves.
I found a replacement head for the V4073 but at GBP80 <http://www.onlineplumbingsupplies.co.uk/acatalog/Motorised_Valves.html it's GBP15 more than the complete valve assembly from my local Screwfix. However, I can't see exactly how to remove the head is it supposed to be held in place by 4 screws, one at each corner? (in which case only 3 were used when it was fitted). Is it really just a case of setting the lever to manual, undoing the screws and lifting the powerhead off and replacing it? Have I diagnosed correctly? Is there anything else I should be thinking about?
(...and, errrm, are these wires running 240V AC? and where are they likely to be getting fed from (I don't really want to have to disconnect mains power to the whole house.)
Many thanks people, from a shivery house this morning. :(