I've tried the official gas safe site and also found some info on a boiler site so I know gunk building up can effect how much the bill comes to, but I'm not able to find anything that tells me how the boiler might be set or for what property size.
Maybe it's something you need to be qualified to do, which is fine...I don't want to be messing around with gas. If it's something joe public can find out I'd love to know though.