There are two or three possible answers:
Hot water sensor is fauly:
CH filter is blocked:
Domestic Heat Exchanger is blocked:
Depending on the installation (ie new boiler on old system and the system not being correctly cleaned during install), the latter two are the most likely possiblity.
If it is a new boiler on a new system (cleaned correctly) the sensor becomes a possibilty.
Carefree
July 2010