Give the top of the zone valve a good hard clout with your hand.
The zone valve will also have a spring loaded manual arm that should return the valve automatically. The valve has probably stuck without closing properly.
Try switching the heating on an off a few times also.
Thanks for the reply. I moved a little arm from one end to the other and left it there. Not sure what that does but the rad's are still on after an hour!
Anything else to try?
To ensure I have the right valve there are two of them in my airing cupboard, one connected to the hot water tank the other not! I assume the one NOT connected to the hot water tank is the central heating?
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