Update: Followed advice and contacted Building Regs by phone. Was told that, although flue was probably wrongly sited, I would have to take legal action myself. I then emailed the same department and requested written confirmation of their advice so that I could take legal action. Within ten days they made a site visit and I received an email informing me that the flue was illegally sited and that my neighbours had been instructed to remove it within a week. (That was two days ago, so we shall see what happens!) Just goes to show what you can do when you force council departments to put things in writing! Many thanks to all for your help.