Central Heating Dilemma?

ok I have a Potterton Mini Minder E (MM)to control when I want the water (DHW) and Central Heating (CH) on and off. A Potterton PRT100 Thermostat and a Myson MPE322 or MPE328 motorised valve (MV). The CH is switched off on the MM, however, when the timer for the DHW kicks in the CH comes on as well. The 2 lights for the CH and DHW on the MV are also lit up! If I select the advance option on the MM to switch off the DHW the CH turns off as well, also on the MV?! Ive tried resetting the MM but this hasnt worked. Not sure which one or both are broke or whether its something completely different. Help would be very much appreciated. Thanks.