I have had a similar problem with my new combi condensing boiler, no problem with the C.H. but a very erratic supply of hot water particularly when C.H. not on.
Eventually sorted it out after contradictory advice from B.Gas. Try running the hot water tap at high speed, this might be all you need to do to activate the cross-over switch from C.H mode in your boiler to hot water supply. It has something to do with the pressure from the hot water firing up the boiler, too little pressure and the boiler does not recognise the need for hot water. Once the hot water runs through the tap, reduce the pressure a little. Failing that, it might be worth having your boiler checked out in case you have a faulty switch inside your boiler.
Hope this helps.
Sam's Nan
April 2010