Just an update to close this thread. I have now replaced the detachable
actuator (£10 from e-bay!) and the problem has been solved. I didnt
need to replace the 2-way valve.
Thanks to all who gave their advice on this
Cheers
Bole

Bole wrote:

The Natural Philosopher wrote:
Bole wrote:
Ed Sirett wrote:

Sounds like the lever is bent (or some other trouble) so that after the
valve has correctly opened it does not shut of it's own accord.

The honeywell valves switch on at the end of the travel (beyond the manual
latching position) other make(s) switch on before the manual latching
position.

You might have a sticking valve on the plumbing side.
You might have a defective valve actuator.
--
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at http://www.diyfaq.org.uk
Gas fitting FAQ http://www.makewrite.demon.co.uk/GasFitting.html
Sealed CH FAQ http://www.makewrite.demon.co.uk/SealedCH.html
Choosing a Boiler FAQ http://www.makewrite.demon.co.uk/BoilerChoice.html
Gas Fitting Standards Docs he http://www.makewrite.demon.co.uk/GasFittingStandards

Thanks
the motorised valve is a Drayton ZA5/679-2 Zone Valve
do these valves/actuators often have problems?

I had one (honeywell) stick on my UFH...boy did the house get
hot..exactly same sypmtoms as yours..manual working sorted it a few
times. then it didn't. Replaced the whole head..not expensive.

cheers
Bole

Thanks to all for your help, the gneral consensus seem to be the
motorised valve or the actuator. I will order both and replace the
actuator first, if that dosent work I will change the valve as well,
hopefully that will solve the problem.
Cheers
Bole