Central Heating Comes on, When Hot water is Turned On ?

I have an old system (circa 1992). I think its is convection pumped system. I have to have the hot water turned on all the time I want the heating on.
Tonight, the hot water went on (on a timer) and the rads started working too. The system has a Sunvic Motorised Valve installed - is it this that is faulty and is it easy to fix? Thanks for your help. Phil.