- The slopey ceiling is made up of 2mm plaster/9mm plasterboard/50mm
polystyrene/50mm Kingspan/0 sarking/clay tiles.

- Flat ceiling is 20mm lath-and-plaster/200mm Rockwool

- Vertical external walls are 2mm plaster/9mm plasterboard/50mm
polystyrene/50mm Kingspan/200mm solid brick

It won't need a very big one, unless the room is huge. You didn't state
dimensions.

Christian.