Would you recommend cutting these 2 stones that span our cavity so there is no thermal issue? Is it a massive issue or fine to insulate and board over?
I would eliminate thermal bridges like that ... they're likely to introduce condensation and mould on the inside wall.
Thanks for your input. Don't suppose you have any thoughts on space under our window cill. Mortar or foam? We're pointing anyway so it's not about ease.
I usually bridge the gap between the two wall skins with two layers of 22mm thick chipboard 'window sills'. It is a reasonable insulator so makes for less of a thermal bridge than most other materials. It also makes a good surface to tile on if you're tiling, a good base on which to mount wooden window sills if that's what you want, or you could even lay a panel of plasterboard on it and skim it so that it's part of the wall. I would fix the chipboard panels with an expanding foam in the gap.