Roof insulation in a house with no loft

[Blix]

My house is a barn conversion and uses the original roof timbers. The upstairs rooms are all attic bedrooms with dormers. So we have no loft.
There are some "loft" spaces here and there but most aren't accessible apart from one bit with pipes.
As far as I can tell there isn't any loft insulation.
The house is difficult to heat because the rooms are large but upstairs is much warmer than downstairs in summer and winter. (It's a sauna in summer).
Is there anything can be done to improve insulation with minimal mess / disruption? What tradesman would I need as I don't do DIY.

When was it converted? It's extremely unusual and wouldn't get sign off with no insulation between the rafters. Usually it's a thin, specialist product. Are you certain there is none?

I'd expect it to be freezing cold in winter if there really wasn't any.

1980s. There is something between rafters and roof tiles I think. Some kind of felt.
It's more the void areas I'm thinking about, for example between dormers. There are also areas behind the bedroom walls. If I had been sharper at the time I would have had these made into storage space.

These should have been insulated with some board insulation like kingspan when the conversion was done. The only way to retrofit is to redo the roof or take internal ceilings down and have them put back up. Disruptive but not as bad of a job as you might think and given energy prices, might be worth thinking about.

Apologies for jumping into thread, but for a Victorian building with no loft space (except tiny top triangle bit, is the way to insulate really to re-do roof, or take down ceilings? And then, if ceilings down, have to re-do window / door frames?

Dreading winter!

How else do you think you can access the space between the tiles and the internal ceiling? It sounds awful but makes a huge difference in terms of warmth and coolness in the summer. I guess the windows and door frames thing depends on how they fit against the ceiling as to whether they would need redoing.

Also worth bearing in mind that you may lose some head height in order to put maximum insulation in.