Bedouin wrote:

What's the best thin insulation available?

I need to find thin insulation (5-6mm) suitable for sticking (using
waterproof glue) to a gently curved surface. This is in a situation
where energy is in very short supply so every little improvement helps.
I expected to be able to find PUR/PIR closed cell foam in roll form but
no luck.

Ah. If you key in 'depron' on an ebay sarch, someone is selling depron
sheet (expanded polyproplyene) in small quantites. 3mm and 5mm thick.
Use water based contact adhesive (copydex?)

Polystryene sheet is also available, at builders mercahnts, as are cork
sheet and bubble wrap, Mock not, these all work.

you can strip standard EPS foam with a hot wire or indeed a wood saw if
you don't mind a bit of mess.