On Jul 24, 9:57*pm, Fred wrote:
I've been retrofitting celotex to a room. It's gone well except for
one slight problem: the window reveal (I put 25mm celotex inside the
reveal).

AFAIK industry practice is...
- 12mm Celotex with 9mm PB over the top
- 20mm Polystyrene + PVA + plaster skim

You can use Marmox 6mm 10mm 12.5mm 20mm...
- Cement & glass-fibre mesh on extruded polystyrene + plaster skim

Marmox is actually better than expanded polystyrene re waterproof.