POTTERTON PERFORMA 24 NO HOT WATER BUT CENTRAL HEATING FINE?

as soon as the hot water tap is opened the pressure on the boiler increases 0.5 bar to 2.0 bar. and the boiler fires. it then produces very little hot water more cold than hot and then the burner goes out after 2/3 minutes this keeps on going on........some say the diaphram needs replacing and some say it is a fault with the divertor valve any ideas??????