baxi 105e flame problems?

i can get domestic hot water from the tapes, but when i turn the central heating on, the boiler goes through all its checks starts the flame then goes out after a couple of seconds. checked the pump, over heat stat, flame sensor and they all seem ok.

please could someone help, but please dont answer if your telling me to call Baxi. i want to fix it myself, if i can.

cheers Dave