main combi 30he flame failure light?

Hi Has anyone got an answer to this problem with the above boiler? when the central heating is swiched on the boiler fires up and runs quite happily for a few minutes (until Temp indicator is up to 60 degrees then it cuts out and the flame falure neon flashes. if I then reset it the boiler fires up and then runs quite happily for the rest of the day. The problem only occurs when heating is initially called for either manually or timed. DHW works perfectly all the time. I have checked condensate outlet, no problem is apparent there and it is not likely to be frozen up. The problem is not even intermittent, it follows the same pattern each time. My problem is that I cannot use the timer to bring the heating on if I am not there as it only continues to work if it has been reset. Any suggestions please ??