No heating - 3 port motorised mid-position valve?

I have a Tower Mid-Position Valve and although I can get hot water my heating will not come on when we switch the programmer for heating only setting. The only way we can get the radiators to heat is by turning the hot water switch on the programmer to 'on' and also turning the Auto/Manual leaver on the valve to 'manual'. The only thing with this is that when the hot water reaches the temperature set on the cylinder thermostat both the water and hot water goes off.

We've had an electrician look at the wiring and can't seem to see a problem. Has anyone else experienced this, or does anyone know a solution. I don't know if it's the valve that's faulty or if it actually is a wiring fault. I know that it is quite common for these type of valves to get stuck, but not sure if there is an easy way to solve this or if we just need to replace the valve.

Any suggestions will be greatly appreciated.