Hi Tony. you say you have replaced the burner unit. is this for a new one?
I presume its not new eg a year old as it would be under warranty.
Check the baffles and the supports that the baffles rest on. you may find that they are blocked or damaged and not letting the gas escape this will choke the boiler.
you could also try putting in a delevan W nozzle the same size as you normally use. i use them on problem boilers sometimes.
Brian Emian Plumbing & Heating Norfolk
April 2011