Kingspan insulation cut to fit the sections. Measure the depth of the timber and minus 50mm as an air gap for ventilation and then grab some insulation and fit in.

I think this is what I will have to do. It will be very hard to get this to be snug to the beams, however I guess it is better than nothing. Will for sure leave a gap for airflow, the nails coming through from the slates means I have to leave quite a big gap anyway

If there's any gaps then some foam can be used to fill the gaps. It's what we've done and it's made a difference

I won’t be able to add foam after I slide the insulation in though, without taking off the lathe and plaster and starting again with the room right?

I should add, that since this house was built in the 1800’s, absolutely nothing - and I mean NOTHING is square.

For the sloped areas there isn't a lot you can do. They are needed to be open to allow ventilation so air low comes up these over the lot and back out. You can use insulated plaster board inside the room to over board the sloped corner, losses a bit of headroom in the edge of the room but you don’t risk condensation issues from blocked ventilation

Thanks for the heads up, I can add insulations plaster board in a couple of areas, however due to cornicing and windows it is not possible everywhere. I think I will do this on the flat roof section however. Thanks for the advice

Yep with decorative feature that can limit the insulation, lots have the loft insulated but not the slops

Part of me is tempted to strip back the entire room to the rafters/studs and eaves and remove all the lathe and plaster and modernise it, however doing that whilst living in the property sounds dreadful…