Vikera boiler won't stop heating the radiators?

My Vokera Linea Max, floor standing, keeps pumping heat into the radiators day and night irrespective of the timer setting and even though I have turned the dial to Hot Water only. (The dial has 3 settings - HW / Off / HW + Heating)
Over the past 6 weeks I have had no fewer than 5 service engineers have a look at it and they have replaced numerous parts but still I cannot have hot water without also having the rads on full.