Celotex for sloping area of pitched roof.

If I understand the question correctly; The internal slopes of the roof had insulation board applied, but has fallen off. If the insulation was friction fitted then re-applying it and mechanically fixing into place (screw though board into rafter) will resolve the problem. In respect of the flat ceiling, it would be best (if head space allows) to under-draw the ceiling. In other words use a insulated plasterboard and fix it to the existing ceiling creating a new ceiling. If needed, it should be taped and plaster skim applied before painting. Removing the existing ceiling is a more significant undertaking, but appropriate, if the under-draw method, suggested above, reduces head height below applicable regulations.