i use 50mm kingspan fixed on with Everbuild Pink grip dry fix foam, then with plasterboard fixed with same foam. All joins on insulated board sealed with insulation tape to help stop moisture getting through.
You can also mechanically fix (some say to do this in case of fire ... but I don't bother - if a fire is so hot they melt, then the room will need redoing anyway! - makes sense in corridors in flats etc for access).
You can use any thickness board, the thicker, the warmer. But some people say that if you put too much insulation on internal walls, the stone walls get too cold and you will get problems with more damp / moisture in the walls - the idea is that it is not perfect, some heat still gets through as old walls "designed" to breath etc.
Here's the pics, was a full refurb. Removal of chimney, insulating under floors, rewiring electrics etc. First time doing something like this for me (my background is office work!).
https://photos.app.goo.gl/tSGFt3HD2CWY7sGa9
Wow that room looks amazing - wish mine looked like that.
So am I right in thinking Kingspan then plasterboard. Obviously the walls would have to be plastered on top of boards again wouldnt they. So would you suggest a 50mm board would be enough? Also what would the thickness be of all the extra so thats kingspan and plasterboard. Just wondering how much I am going to lose of an already quite small room.