Worcester Bosh 24SI II?

When boiler initially switched on it ignites and the heating and hot water works. After about 5 minutes (has been as long as half an hour), red light starts flashing approximately 4 times per second and shuts the whole system down. We have done the reset procedure with still no joy, have checked all around the flue no problem, fan and pump appears to be working. Could anybody give some light onto this problem as now into day 3 with no heating and hot water