Jay J3 wheelchair seat back (mid-thoracic, mid-contour), with the mounting hardware, for £51 off eBay (that's including £10 P&P). Normal retail price £506. I saw it at £29 with 3 days to go and thought 'that price has got to go up', but it never climbed higher than £34, so I sniped it with 20s to go with a bid for up to £55*, and a last second counter-bid only pushed my bid up to £41. Utter bargain.
* I'd have gone higher, even waist-height J3s normally go for about £150 on eBay. but the pics weren't clear on the state of the upholstery, and the description only gave width, not precise model. Parcelforce just delivered it and the upholstery is about as I expected, used and a bit fuzzy in places, but should perk up with a turn through the wash - I just downloaded the owner's manual to check settings.
If I was buying new, I'd probably go shoulder-height and deep-contour (a snip at £572), the same setup I tried on kaberett 's chair. But for now, this is an incredible bargain.