Worcester Bosch Highflow 400 BF boiler problem?

Hi, my boiler produces hot water on demand OK but refuses to turn off the central heating. Even with both heating circuits turned off and the boiler switched to 'Hot Water' only the radiators still get hot. Not only is this a little wasteful on the hottest day of the year the house is a little uncomfortable. Is it likely to be mechanical (heat exchanger water diverter) or electronic (PCB)?

Any comments I would be most greatful.