If you have a separate circuit/control to heat the hot water, then it is most likely controlled by a motorized valve. The control sends a signal to the valve to open (and turn on the boiler). These do fail and can be replaced.
Is there something like this visable in the hot press?
To be honest, we need a bit more information to make a more educated guess.
1. Is your heating zoned, i.e. is there a separate button on your heating control for hot water?
2. What happens, if 1 above is true, does the button light/ display say that the water is heating?