Halstead Ace High Boiler - Burner on light goes red when trying to run hot ?

I would be hugely grateful if someone out there could help me with this.

The CH function of the boiler is operating perfectly well however when I try to run hot water through the taps, the boiler ignites and creates hot water for about 30 seconds before cutting off. The burner on light turns red and there is no hot water. This can be fixed by pressing the reset button under the boiler but it is only very temporary as the fault will re-occur if I try to run hot water again.

I have had two engineers (British gas and Aspect)out to look at the boiler and they just press the reset button under the boiler and then say it is fixed which is hardly worth the £100 they charge for the call out.

I am goin mad here so any help anyone can provide would be greatly appreciated.