CH and DHW running when switched to timer off and DHW only???

I have a Britony Combi 80 (chaffoteaux) that the Central heating comes on when switched to DHW only or both! It ignores the timer, and the pump constantly runs. Has anybody seen this before. Looking around on the web, people say the three way valve is poor on this model, but that would leave me with either DHW or CH, not both? Any help much appriciated