HURRAH for having found something appropriate! I've gone and sprung for that one I linked you to; I have a friend picking it up for me and posting it on, hopefully >>; (Should fit me better than the current dance chair, is the idea...)

Lots of fingers crossed for it arriving ASAP; I look forward to hearing how it works for you!

Hmm. It looks from https://www.quickie-wheelchairs.com/replacement-parts/sunrise-medical/jay-cushions-backs/jay-backs/jay-j3-back/mounting-hardware/j3-back-hardware as though you'd need quite a lot of mounting hardware -- if you're going with the Sunrise stuff rather than jury-rigging something, you have parts 1, 6, 8 and 14 on that back -- but none of the rest of the mounting kit. Might be worth messaging the seller to ask if they're including the mounting bracket and pins, or whether it's just what's photographed?

Iiiii'd honestly suggest getting in touch with Gerald Simonds or EPC to ask how much they'd charge for mounting hardware for a back, but it looks like it would be another hundred quid :-s Like, it's preeeeetty cheeky to say "backrest retails for £520" while leaving out that a lot of that is mounting hardware...