Hi, I thought I'd posted an update but I don't think it got through.
I have a chimney that runs up through the middle of my house and on monday I finally summoned up the courage to knock it down. I made a hole through the tiles and then just went at it with a hammer and a chisel. Then I extended the battening put some new tiles on and job done. It might not necessarily comply with current regs but it seems good enough.