Central heating not working independantly?

Replaced room thermostat as old one damaged, replacement is Drayton Combistat, 3 wire config'.
All seemed well for a while but now heating does not work independantly from hot water.
system is not combi so is this as simple as wrong type of stat or is there something else i should look at.
Checked wiring all intact with power to unit, contacts clean and power in demand/ call for heat wire.
Thanks Namobma

namobma
December 2009