Insulating semi pitched ceiling

[Spaniel87]

Just wondering how best to insulate this pitched section highlighted in red. The whole ceiling is boarded with what looks like tongue and groove and then plaster board over the top. The flat section is insulated above with loft insulation, the the pitched section has no insulation as there is an air gap. Can I just put some celotex or something on top of the plasterboard on this pitched section and then just board over the top. Or is there another way of dealing with this. Any advice extremely welcome.

 

[Cubist]

Some kind of insulation of that space may be useful but, in my opinion, its greatest contribution would be to minimise the risk of condensation forming above the radiator due to a cold spot. Safe to assume you mean to install the celotex/whatever in the loft space and behind the visible plaster?