The Halstead Finest Gold is renouned for the flue pressure sensor caking up with silicon from the gas supply, they say all boilers suffer but I have asked around and can not find any other make with the same problem.
You need to remove the flue fan, inside the nozzle there is a ventura with 2 plastic pipes connected to it. remove it, clean it with wire wool and replace. this might be your problem. the new ventura is teflon coated or something so is better but still not perfect. I do mine 3 times a year.
bigredrep, April 2008