Worchester 24i junior boiler problems?

I have a Worchester 24i junior boiler. When the heating has been on for a while (this can range from 15 mins- 2 hours), the heating cuts off and the red warning light flashes really fast. When I press the warning light to reset it, it then just starts to flash really slow. I then have tried turning it on and off again, and pressing the button to reset, but it continues to flash for ages and I then have no heating or hot water. The engineer came out and apparently couldn't find anything wrong!! Please help!