24i/28i Junior Problem?

Father christmas has brought me an early present! Please help if you can! My boiler knocked off while in use and I got the slow red light flashing. When I try to reset the boiler mostly it doesn't work. I have got it to restart a couple of times, using only the reset procedure but it only works for approx five mins before knocking off again. It does not seem to be an increase or decrease in demand which knocks the boiler off i.e turning on or off hot water taps/radiators/shower. Just knocks off after a few mins. Mains water and Gas feed ok. Spark ok. If anyone has any ideas I would love to hear from you, many thanks...