How to best INSULATE a sandstone/slateroof house please?

[54isanopendoor]

I am in a Victorian house with 600mm sandstone walls & the original slate roof.
It LEAKS heat.
It's been assessed as category Fsomething for energy efficiency.
What can I do to improve it please?
I may be able to get a grant towards interior insulation. It might be as thick as 6.25mm which would completely ruin the internal features (cornice, dado, skirtings, shutters, door archtraves etc). I might be able to get 2.5mm which probably would look less awful but be less effective). It will also cause huge upheaval (I'm disabled & house is cluttered & no help to clear for works).

I'd prefer exterior insulation (not listed, not a conservation area) but I can't seem to get help for that.

Can anyone advise please:
If modern insulated plyboard (silver backing, then 600m of yellow stuff then plyboard) would cause any long term issues in this kind of house?
If it would be ok to use in the room in the roof areas or in the basement (half below ground, half above, with windows but does let water in the floor up to 3cm deep 2/3 times a year...)

thank you.

bump

anyone ?

bumping in case anyone can advise ?

When it was given the EPC rating (the F) were you not given any recommendations on how to improve it?

You need an energy survey with tailored recommendations. The surveyor should give you options for insulation and how much impact it should have on your house's energy performance. Google "home energy assessment [your area]" or have a look here: www.gov.uk/green-deal-energy-saving-measures/get-an-assessment

with period propertiees not all types of insulation are suitable as house needs to breathe anything that seals mositure will increase your problems not decrease them
ideas
secondary glazing, shutters, real thick lined curtains including door curtains which can be on special rods (portiere) so you can open doors with curtain closed
wool insulation in loft ( not fibre glass or spray foam)

@CatJumperTwat No. I called 3 diff Co's that are offering help to those on certain benefits. Only 1 was backed by the Govt. They came out & I was asked to sign an iPad to give permission for them to check my benefits. I did. Was told I'd be given copies. I wasn't. I've since become aware that they apparantly did an EPC that day. No recs given. Nothing in writing except my house now has an awful EPC attached to it. I've complained ....

@Cottagecheeseisnotcheese

real thick lined curtains including door curtains which can be on special rods (portiere)

THANK YOU for this! I live in an Edwardian tenement and have been trying to sort this out for years, but didn't know what it was called - amazing, have ordered one now.

I thank you, my cats thank you, my bank account thanks you, the planet thanks you.