I have no idea what may be wrong but that has never stopped me offering suggestions...
The boiler is stupid - all it can do is heat water - it does not care what the hot water is for - your boiler works if it heats water.
An external valve will direct the water to the radiators or to the hot water storage tank.
It may be that your external valve is stuck in the radiator position - but I know nothing - I just guess.
Good luck...
July 2015