How can I fix my heating?

In the morning our heating comes on as we have programmed it to but in the afternoon it doesn't work. The light 'CH' is iluminated so the programing is correct but there's no heat. If you turn the termostat to a higher temperature normally it fires up, but nothing in the afternoon. The boiler is a Glow worm Ultracom 18sxi. It just seems really strange that it works in the morning and not in the evening - I have checked all of the obvious things like 24/12 hour timing etc.