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danb
OK!
The best insulation layer for active use (i.e. wearing whilst you're cycling along) is fleece, or lightweight synthetic, emphatically not down. Down is for wearing when you're stopped, or cycling very very slowly, or in your sleeping bag to improve its warmth.
Both fleece and synthetic materials retain their warmth when wet. Down doesn't, and claims of "treated down" or "DWR Down" or "Water-repellent down" are 90% bullshit. A little bit of rain is no problem for most down jackets, but if it's sustained then they become rather useless.
For fleece, you want a lightweight material, 100-weight. This is "thin" fleece. You can get a £5 old Karrimor/Lowe Alpine fleece, or a £100 Arcteryx one, they will perform near identically. I prefer them without any pockets, hoods, or thumb loops.
For lightweight synthetic, anything sub 300g is ok. But the best is the Patagonia Nano-Air Light. It has received rave reviews as an active insulation layer, unfortunately I can't afford one yet.