"710 approx to meet hetas regs", this is new to me, and explains a lot of posts similar on Houzz lately. I moved out of a rental that had nowhere near that with a log burner in, and in fact nowhere near the depth youre trying to achieve! I think I would have said sod the regs and put it how you want, swear it was always like that lol. ah well... I'm sure there is a solution, somehow....

Firstly a general builder can open up the Chimney and get rid of the tiles and the half height mantelpiece ( what were they thinking?? ). Secondly get a Chimney sweep in to check over the Chimney with a camera and advise if it is suitable for a log burner. You may find that companies that sell log burners will do a site visit to check out suitability too.

Thank you ED. It had loads of ring marks before, which were horrible (why would you leave tea/coffee cups on a lovely old fireplace ??!! ). Thankfully most of them have been scrubbed off. I’m really grateful for your comments,

While definitely being of "it's time", it is a rather unremarkable piece and while I'm the first to advocate preserving features, even I would struggle to justify it here! Personally, I'd replace entirely with something of a style/period of your choice, hearth & all. There are many much more attractive pieces, both modern & traditional. I'd encourage you also to look on Ebay - it never fails to amaze me how many really beautiful fireplaces are always for sale at a raction of the price of new. Some great bargains to be had.