Yeh, thanks for the report back. When I started reading it I was kinda hoping it was going to be a continuing disaster story - that's called schadenfreude by the way, a German word for taking pleasure at other's misfortune, but BGs misfortune rather than yours!
Your point about your low gas bills does indicate how inefficient your old back boiler system was. However even a new back boiler with an upgrade of the controls might have resulted in significantly improved efficiency. On the boxing-in of the flue, you may remember being advised that it should be possible to inspect the whole length of the flue. Can this be done (removable panels, etc.)?