First check the pressure, should be 1.0 - 1.5 bar. Just in case.
Do you ever see the big blue light flashing?
If not, and the hot water just doesn't stay on, it's probably a problem with the flow turbine. It's cold water flowing through this that tells the boiler to fire up in hot water mode. Not really something you can do yourself, job for an engineer.
Bunslinger, April 2009