Hi Tom, yes it's pretty rare, it does come up occasionally on ebay, and fetches ridiculous prices.
I recently found a box of sewing related materials in a local charity shop, and amongst it was seven full cards of the original Chadwicks wool, unfortunately none of them were 477.

I bought them all for 50p each, as they were nice suttle colours, and may come in handy.
I also told the lady in the shop to keep her eyes peeled for a card of 477, and said I would make a hefty donation to the charity if she found one.
I did hear that every demob suit had a card of 477 in the top pocket, for darning purposes, so ask around, must be some somewhere.
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Ti's here beneath this sturdy bridge,where bats and moths do fly,
That the wily trout of the smallest size, against a rock doth lie.
SKB tackle supporter. I love Furled Leaders.