What is it? It was just loose in the lower damping leg, but without it the fork seems to lift up about 10mm too much with no spring resistance. Should it attach to something?

I've absolutely no idea what it is, but it definitely looks it should be fastened to something and not floating about loose.

Is it not the bottom out spring?

I don't know the fork but it looks like the bit that does the soft bottom out. Bump stop IIRC
Possibly both a mechanical spring and a hydraulic stop
Just put it back in.

I dunno either but try checking the service diagram

That is a top out spring, stops the fork clunking on return.

Looks like the top-out spring/assembly.

I'm pretty sure it should be attached to something!
Download the service or parts (exploded parts diagram should show where it goes) manual and see what it should be attached to.

Not managed to find an exploded diagram yet…

Don't you know a flux spring when you see one?

"Not managed to find an exploded diagram yet… "

I really think you should rather than just throwing it back in randomly, good luck 😉

ha more replies on here than SDH, more funny than SDH but not as helpful i don't think!

lol too true dirtbiker.

Managed to find a diagram eventually here: RockShox Diagrams

The previous owner had converted the fork from 2-Steps to U-Turn, but had left part of the air spring assembly where the top out spring goes, then plonked this part in the damper side lower 😐

All sorted now and the forks feel much better!