Greenstar boiler heating water but not central heating independently?

Hi folks

I wonder if you could help me with the following problem - I have a Worcester Bosch Greenstar ri (15 I think) condensing boiler (it's not a combination) and it's decided to have the following problem:

The boiler produces hot water and central heating in the normal way but stops heating the radiators when the hot water is...erm...hot. So even though the thermostat shows the central heating needs to be on - the boiler won't heat the radiators unless it is heating hot water for the taps.

Interestingly, if we run the hot water tap for a while, the boiler kicks in and heats the water/radiators. So whether the boiler is heating the radiators is entirely dependent upon whether it kicks in to heat hot water.

Rambling, I know, and sorry for that - but any clues of the reason? The boiler itself seems to work fine.

Many thanks for your help.

Richard