First let me make it clear I dont know I am not a heating engineer and I dont know the model in detail but...
Its probably not the boiler but the divertor valve(s).
Most heating systems have a system of powered valve(s) that switch the heating circuit from Hot Water to Heating.
Some Systems give you either HW or Heat and if you select both will bring the temp up on heat then switch between the two to achieve both. These generally have two valves driven by your controller.
Other systems Give you either Hot Water or Hot water and heating. These generally have a single pipe circuit with the heating element of it (radiators) switched off with a single valve.
Your fault could either be the valve(s) or the controller (programmer) which activates the valves.
