Potterton CH System will not work!?

I have Potterton Suprima Boiler which is heating water only. The CH system is intermittent and only occasionaly works with the time clock. The CH light on the controler is ON but the boiler will not fire up. I have tried pushing the motorised valve to manual and nothing happens. I am guessing this a switch/thermostat problem. Any ideas what I should try next I could do with some heat this evening?