Yep, got the new nylons in, nice tight rotation now. However I still have movement from worn seat runners, could probably do with tapping the edges down to tighten it up.
Some pics, although, if you need pics to do this one, you probably shouldn't be trying it

Remove your seat completely, leaving the seat base (and all its dirt) visible.

Unbolt the 4 x 13mm Nylocks

Lift the circular centre off, you need to twist it to get the release lever out from the hole. 4 of the nylon parts are on the seat base, pop them out with a screwdriver.

Lift the second base off revealing the 4, probably more worn, nylon bits (and more filth). Good opportunity to clean things up a bit as well.

These bits come out very easily, either lift with your fingers or a small screwdriver.

Old and New

Old and New

Old and New

Insert your new bits

Insert your new bits - maybe a little time in hot water, or a tap with the hammer to get these ones home. Making sure to put them in with the curvature the correct way.

Put it all back together, make sure that it gets bolted down squarely.

Find the correct tightness, between no movement and being able to swivel - seemed to be get to tight, then back off 1/4->1/2 a turn.
Very handy if you wanna replace missing parts, reduce seat movement (do check runner play).
Ian will be selling them through his business if anybody needs a set. Nothing to do with me, I was just making sure they fit (they do!),