In reply to Duncan Beard:
Two options from my experience as someone who can get cold very quick when inactive. Sorry, neither are low budget end, though I would definitely buy either again if I ever needed to. Probably available at reasonable cost 2nd hand though if you don’t mind that.
The warmest synthetic gilet I’ve had and still have is a Paramo Torres. Mine is some 15 years old and is the 133g fill one. Designed for over layering, but I use mine both under and over depending on circumstances. Latest ones are only 100g fill now I believe, but some of the older ones will be about used if you don’t mind that. Not stylish, but practical and mine has been lasted really well, though has had three minor stitching repairs over the years due to the abuse it has had to put up with.
Warmth for the lightest weight I’ve experienced is a Patagonia Micro Puff gilet. As you would expect it is certainly not cheap although I did pick mine up in a sale many years ago, but in my opinion is very warm for its weight/thinness.
Post edited at 22:31