Rosebery - The system is a fully pumped one. This problem has only started occur. Before there wasnt a problem when we just wanted the CH on, on its own.
Steve the gas - I've been informed by BAXI that the Prima 30B is a conventional gas boiler. In the airing cupboard upstairs (where the tank is) i've noticed that there is a small unit attatched to the pipe work near the Grundfos pump. I think this is the diverter unit.
What puzzles me is that this problem has only started to occur. As I have said, before we were able to turm the CH on without having to turn the HW on. However now, when we turn the CH on via the programmer there is no response from the boiler (pilot light burns as normal) BUT as soon as we turn the HW on via the programmer (while CH is still on via programmer) the pilot light in the boiler roars to a brighter flame (and the radiators start heating up as expected)