Zone valve not communicating with boiler?

Hi
We've recently turned the central heating on. The hot water has been heated by the system all summer, no problem. With the central heating on the boiler burns gas then just before it stops burning gas there is loud banging. I have tried playing with the zone value and the thermostat: If the thermostat says the temperature is too low the zone value whrrrs, the boiler burns gas the pipe either side of the valve gets hot. When the thermostat thinks the temperature is reached the value whrrs and the boiler makes banging noises. If while it is banging I manually open the value it stops baning and carries on burning gas. My guess (which I accept may be completely wrong) is the zone value has not told the boiler to stop burning gas or the boiler hasn't responded the water in the boiler is then static and boils - the banging is bubbles of steam being relased - the boiler's own thermostat then tells the boiler to stop burning gas.
When the time clock has turned the central heating off the boiler continues to bang every 4-5 minutes. To stop it overnight we have extinguished the boiler.

Any ideas what is happening and how to fix it?

Thanks

I think your diagnosis and reasoning is good

Sounds as if the microswitch in the zone valve head has stuck on.

Replace head.

Thanks for your quick reply Dave, guess I'll have to go to B&Q (or can you recommend a better supplier) for a new head.

Thanks

I would visit a local plumbers merchant, eg Plumb Center, PTS etc, before I ever considered a shed