Just bought a small 2-bed flat on top floor of a stone Victorian house. All walls seem to be lined with lathe and plaster. DG timber sash & case windows/veluxes. North half of the flat (bathroom, kitchen, stairwell) has a pitched roof with coombed ceilings, the south half (living room + bedrooms) has a flat roof. There are adjoining properties to east and west but with slightly lower gables.

Open aspect front and rear so south side should get sun all year (as much as we get sun in Edinburgh). Ownership of the roof is shared with the flats below. Building not listed but in conservation area.

Apparently there is some form of insulation in the flat roof, but I don't know what. The flat roof was replaced about two years ago after blowing off in a storm so it is probably modern but also probably quite basic. I don't know if there's any insulation in the pitched roof.

Heating is new-ish gas boiler with high pressure tank. (I'd prefer a combi-boiler but that's really do to with doing away with the tank and gaining the space as storage).

Any thoughts on where some money might be best spent to improve comfort/reduce energy bills? Or should I just wait and see how it feels during the winter before making any decisions? It seems cosy enough at the moment - and I'm not living there yet so no heating on.

Start with draught sealing and then do insulation, try to gain access to any loft voids by cutting in trap doors or fitting panels

There is a lot of information on here about insulating "skeiling" or sloping ceilings.

Happy insulating

Until 31/10 easy to areas can be done for free under the CERT scheme.

Having previously lived in a top floor Georgian flat in Bath for many a year I would agree with the above comments and as Tony says I would be initially concentrating on dealing with the draughts. The sense of comfort achieved when we comprehensively dealt with air movement through the flat meant that the thermostats were not being constantly edged upwards. For us this involved removing all the (sash) windows, which required refurbishing and fitting draught strip brushes to all edges, not sure if you have sash or what type but if pulley you can, and should, try to proof the cord wheel as I found a huge amount of air funneled through this opening. Floors, huge amounts of air movement through ours until sealed. From there on as per Tony and Tim. We also reinstated our shutters, which for a building where there were historic limitations to upgrades, was the most worthwhile improvement. It provided additional protection from draughts and heat loss through glass (single glaze), in winter months and all nights cut down excess solar gain in the summer and was again an improvement that provided an obvious sense of wellbeing.
We were able to insulate the walls internally which brought improvements and helped design of the shutter box and we used EPS.
Redoing it now I would investigate useing a more breathable material such as Pavatex but during the time we occupied the flat there were no obvious signs of damage to structure, having said that the structure reasonably dry and I didn't do any retrospective investigations behind the insulated external walls.
I also know that the ownership of roofspaces, their use/change of use and use of external roof area itself can be fraught with danger, From a reasonably long term occupancy I can say its a really worthwhile aim, hope this helps and good luck.