Worcester 24i junior combi boiler hot water problem?

I have a Worcester 24i Junior combi boiler, that is experiencing a problem with the hot water. If the heating is off, or has only been on for less than 5 mins, attempting to get hot water will result in the boiler failing and the error light coming on (flashing once per second). If I then reset the boiler and switch the heating on for a few minutes, the hot water will then work fine.
My limited understanding of the boiler leads me to believe that it is not igniting correctly under the situation above, however I have no idea why. Can anybody shed any light on what might need to be fixed here?

Duncan
February 2013