John wrote:

"Jim Thorogood" wrote in message
[snip]
At the moment the boiler
ignites and burns for a few seconds but the the green light continues
to flash at high frequency instead of going on continuously as it
should , the burner then extinguishes.



Amongst other possible causes - Check for a break in the ignition/flame
sense lead. A gap will permit sparks to pass but block the sensing current.



Yes this is the most likely cause. I had exactly the same problem with
mine a while ago and replacing the ignition/flame sense lead fixed it.