On advice from Adam Cron I moved one of the rafters to get out of the way of the flue. He also reckoned that the hearth was too big. I hadn’t really thought about it, but it was. So I made it smaller. Also banged some more corrugated iron up to protect the walls near the fire. Cronsai talked me right through the process of putting the chimney in, so I’m confident about next steps now. He also gave me an amazing old had which will fit both flues nicely. I also put up a few more pieces of lining.