Roof Insulation for Victorian Top Floor attic rooms

[happyduck]

Hello

I live in one of the typical Victoria Terraces on the Camberwell/Peckham border. Our top two bedrooms are up in the roof with casement windows. However, I don't think it's a loft conversion. I think the house was originally built with the rooms like that as there is absolutely zilch insulation - we had some replastering done when we first moved in and from what I remember it's basically plaster on lathes and then the boards for the tiles to be attached to - which makes the rooms very cold in the winter and is obviously hugely wasteful in terms of energy.

Has anyone got the same problem and have you managed to solve it. I'm looking for suggestions for an economical and non-intrusive way to insulate these rooms.

Also, I need to get one of the sash windows in the casements replaced so any recommendations for a good local sash window company would also be great.

Thanks for reading.

Daisy