It will be the ECB, and it is not hard to change, costly though.
When I say it is not hard to change, it is because I have had three opportunities to change my ECB on the same boiler within 3 years, you will find it is sitting at he ignition stage and all it needs to operate is the gas valve to open, and usually the relay which is one of three fails to operate, I am thiking of changing the relays my self and flogging all my old boards.
J Fraser
February 2008