We had a new boiler put in at the end of the winter so are only just starting to use it properly. On the timer it comes on fine in the mornings, and goes off again. But in the evening it comes on but not very hot and kind of drifts off after an hour or so. Is this a fault, or is it because it has reached the 20 degrees it is set on and thus goes off?!
Try what gingebee says, we have ours at 20 at the moment and it's hardly on in the evening, If you raise the thermostat you should hear the boiler kick in almost straightaway if it's working properly.