It is not london, but a modern housing estate elsewhere. I think when planning is done and additional consent is given to extensions, sometimes future boundary issues are overlooked (pardon the pun).
I probably came over as a bit angry the other day, which I was. Calmed down now. It was just that the neighbour assumed I would not mind the flue over the boundary and was going ahead with the work, even though I raised concerns 2 days earlier when they 'consulted' me . I did ask them to check the various regs to make sure ok and expected them to come back to me. I had not expected that I should have to look into the various regs, on the day they had the work planned for.
Anyway, I think they are now having another boiler with the flue through the roof.
Out of this I would say the learnings are.
If you are planning work near the boundary, which is not urgent, make sure you give the neighbour plenty of notice and ask them for their consent, if anything is to come over the boundary. The neighbour might want to check for themselves about any issues, before they offer their consent. Also as the home owner planning the work, it might be a good idea to also check for yourselves. All it would have taken were a few phone calls e.g. Gas Safe helpline, local council building consent people.
Leaving it just 2 days before the actual work is to take place, to just consult the neighbour and then to go ahead with the work even after they had raised concerns is not a good idea. The various regs are just too complicated to leave it until the last minute.
I would have thought that when existing boilers were re-sited, that the GSR person doing the work, would have made sure that all the regs were complied with and any consents were obtained before they did the work. I don't think I would conduct work, putting something over a boundary, if I could be called back later to do more work for free, if there was a dispute later.