Why is my feroli domicompact boiler not working?

I have a domicomapct condensiing boiler which is just over one year old. The hot water supply is working but when I switch the boiler on either timed or manual it will not switch the central heating on. This has happened occasionally before and strangely has kick started the central heating when I have run the hot water tap. Now though this will not work. I've turned the thermostat up to the highest level but still there is no central heating output. I don't want to call out an engineer if this is something simple that I've done wrong!! Any ideas please??