The burner should fire up initially when turning on the heating and then go out. After a minute or so the diverter valve should open and the burner fire up again.
Check you have no dripping hot taps as this will prevent the diverter from operating.
If the boiler is over 6 or 7 years old the diverter may require changing.
having left the boiler in constant mode next day , while breakfasting i heard a number of fire ups then low and behold LIFE ..... must have been a sticky diverter which freed itself.
now there is C/H and H/W .... praise the lord ..........
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