I read the thread and still none the wiser. Legal or illegal, safe or not safe, owner or neighbour responsible, who, what, where, how etc etc.............. [emoji2957][emoji2957][emoji2957]
I would have thought you could extend the flue like a chimney somehow - although the resident gas plumbers will probably explain that is impossible. Failing that I would get a second opinion, and maybe ask Gas safe (or whatever Corgi is called now). Also speak to Building Regs at your local...