AS 'P'
If we do not know the system, We cannot give accurate answers, with more information, we may be able to give educated guess. As 'P' was unable to find info, I am not going to look.
SO what you need to do, is describe the system. IS it a combi boiler, from what you say I would think not. What pump does it have, It will have a three way valve, if combi integrated, otherwise seperate. Control board etc. Need more information to be able to help. Post with as much of a description as you can.
Initial thought, if heating coming on when asked for HW, divertor valve fault
SD, October 2012