I think a long screw with a huge plastic washer under the head. Usually they would be fixed while the wall is being built, there is a special cavity tie that has an insulation retainer and leaves an air-gap on the cold side.
Go to the Knauf (or Kingspan or Celotex) website and see what fixing accessories they list. If not obvious, email the help desk. It may be listed as Drylining, which is probably what you are doing. You will need a vapour barrier on the room side.
I know how to retrofit to timber studs or rafters (there is a long nail with a big head), but not to a wall.
Well according to the internet they are dwarf walls
Chalet bungalow... But yes I guess knee walls... There is a triangular space along side the bedroom that I can crawl into that has a gale force draft blowing into it - that is the side of the house that gets the wind head on, it's on the top of a hill!
Use mineral wool between the joists. Leave the eaves open so you can see most of the thickness of the brick walls.
You can fix rigid foam slabs to your knee walls, and can also slide it into gaps where there is plasterboard. Use 100mm if it will fit. You can use the big-headed nails. They are sold by places that sell insulation. The foam is finer and coated with silver foil, has more insulating power than the same thickness of mineral wool. It is better than plain expanded polystyrene. You can stuff wool into any gaps where the slabs do not fit tightly, and you can get matching silver tape to seal joints.