Baxi Solo 3 50 PF Boiler - Only works when the hot water is set to 'on'?

Hi,

I have a Baxi Solo 3 50 PF boiler. At the moment it is only working for hot water (which it doesn't seem to have a problem).

When I switch the heating to 'on' nothing happens. It doesn't try to start up - just remaining idle with all of the lights off (Boiler, Fan, Pilot and Burner).

I've tried restarting it has shown in the manual but to no effect.

Thanks