What you need is some pure soap flakes. Detergent soap (normal washing powder) is wrong and damages the feathers. No more than 30ºC. No fabric conditioner or wot, and make sure yer soap drawer is clean of any powder residue.
Once washed and spun, take it down to a local launderette, with they big tumble driers. Medium heat. It’ll need about £3-4 to dry it thoroughly. Bung a couple of tennis balls in; they will bounce about in the drier, and help fluff up the down once more.
Done this several times to my 15-year old Rab down jacket, and although it’s a little past it’s best now, it still keeps me warm even when it’s proper cold.
I think it was Nikwax do a ‘Down Wash’ solution, but it’s very expensive and is essentially just liquid non-detergent soap. Rip off really. If you can find something similar but cheaper, then go for it.
Nikwax also did a ‘Down Proof’ waterproofing solution, which was effective for a couple of showers, but then seemed to lose effectiveness. TBh, if it’s warm enough for rain, you shouldn’t really need a down jacket. Snow shouldn’t really ‘soak’ the jacket, but brush it off as much as you can.
Look after a Rab, and it’ll look after you. When this one dies, it will be replaced with another. Top quality product. Made in England, too! 😀