radiators not coming on ?

I've had an engineer out to my house a few times about this problem but still no joy so hopefully you can give me some information.
The heating comes on every day on the timer but only for the hot water, the only way we can get heat into the radiators is to manually fick a switch on the red box at the boiler and it comes on. At the minute he has overrided the thermostat and it just comes on at full heat (which so unbearable at times). He says that next if its not the thermostat (which it's not) he will have to drain the whole system and is unable to give us a price for the job. Can you help with this please.