Mid way valve fault??

I currently have hot water but no central heating
When I move the 3 way valve to the H position manually the boiler fires up and I get Hot radiators
When I switch the programmer to Hot Water only then the valve moves automatically to the W position
If I then switch the hot water off and the Central heating on the mid way valve stays in the W position and the boiler doesn't fire up
It will fire up when I move the switch manually again to the H position
Does the actuator need replacing or could the fault be elsewhere?
Any assistance appreciated

Mick
January 2011