No hot water on Worcester 28CDi?

I have a Worcester 28CDi combi. The CH works fine but the DHW has a problem. When i open a hot tap the red light on the front of the boiler indicating hot water demand illuminates. The red light indicating the flame ignition does not come on immediately, it takes a few seconds then the flame comes on but goes out again after approx. 30 seconds. It stays off for a similar time and the cycle repeats itself. The net result is warm then cold water following the flame ignition cycle. Clearly this is not correct, anyone have any ideas?