Insulation on the roof of the bay window

Is there anyway to remove the plasterboard that is already up? Removing that and then attaching insulated plasterboard to the timbers would be my preferred way. If not, if that plaster up there on it is sound and secure, you could potentially use adhesive and glue your insulation and board to it with supports to hold up while the adhesive cures.

You should of filled the space between the timber beams before plaster boarding now you would have to remove roof

If the bay window has a flat roof then taking the plasterboard off is the best option. Then you can cut PIR board and place it between the exposed timbers before plaster-boarding again. If there is space, you could just add an insulated plasterboard into the existing plasterboard, The best solution would be both removing the plasterboard, installing PIR and using an insulated plasterboard rather than normal plasterboard. If there is a peaked roof over the dormer and it is quite large there could be an access through the sarking boards in the loft to the space above the bay window. More often than not, it will involve stripping the plasterboard unfortunately.

I would clean ceiling first and after apply special chemical to keep moisture away. I’m not sure what kind of ceiling you have in there. If it possible, when you can drill in plywood/metal stud/wood. Put insulation inside and plaster it. Where are many ways to fix it. This is one of them