After being worried that the wiring might scupper the mad thermostat plan, it turned out that the three port valve was the weak link. After a short spell operating everything manually I can put away the spanner now thanks to a new Sunvic MoMo valve. In theory it should be better than the old Honeywell valve since it turns the motor off when the valve is in the correct position, rather than just holding the motor against a spring. The flow through the valve should be better too, so I’m also hoping it will mean I can knock down the speed of the pump. That’s the theory anyway, unfortunately there’s a chance that it will actually be less reliable in practice!
It would have been easier to replace the valve before the airing cupboard was taken over by a very large cylinder…
…so we now have a rather large ventilation hole in the bedroom! (That’s the wall Jo painted as well!)
Adding ‘install access panel’ to the list of jobs, which already included ‘replace airing cupboard door so it can close’ (more on that in a future post)!
Notes from a small field 
I think the Honeywell valve uses a small heating element and thermo-expansive wax like old automatic greenhouse window openers, not a motor.
however I could never bear the thought of 3 Watts per radiator day and night to keep the radiators off.
however I now realise that is only required when the heating is on.
I think some zone valves use the wax method but as far as I know most 3-port valves work with a synchronous motor and a spring– my old Honeywell certainly did.
I’ve read a few comments that using two zone valves instead of a 3-port valve is more reliable, and I’d be inclined to believe that! Unfortunately that would involve more work to alter the existing system (and probably a bigger hole in the wall/wallet!) so a straight swap for a MoMo valve seemed like the best option.
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