Boiler fires up when demand for hot water but doesn't when demand for C/H?

I have a Potterton Netaheat Profile 80e boiler, Honeywell V4073A mid-position valve, Honeywell ST699B programmer, Honeywell L641A cylinder thermostat, Honeywell T6160B room thermostat, Honeywell Sundial Plan wiring centre and a Grundfos Selectric UPS 15-50 3 speed pump.

When there is a demand for hot water the pump runs, the boiler lights and everything works as normal.

However when there is a demand for central heating nothing happens - the pump doesn't run and the boiler doesn't light.

The problem occurred earlier this year and the temporary get-around I used while it was still cold was to set the mid-position valve to manual so that I got hot water round the radiators when the boiler was running for the domestic hot water.

During my investigations so far I have tried a new ST699B programmer but the problem was still there.

I have checked continuity of cables (we had some mice in the loft earlier in the year) and everything seems OK.

I have checked the room stat connections and they are all sound; I have also used a multi-meter at the wiring centre and have confirmed that a circuit is made when the room stat is turned up.

The fact that everything works fine for the domestic hot water leads me to believe the boiler end of things is OK but I am assuming that it is the same circuitry to/from the boiler for both domestic how water and central heating.

Any ideas?

Thanks.

John P.