Removed the red rebound adjuster nut, undid the black nut a few turns, and gave it a tap (protecting the nut with the 10mm socket) in order the free the internals from the lower. Now when I try to undo the black nut further, it just rotates the entire rebound damper with the nut. Have tried compressing the fork with all the air in (and also let out) and the fork locked out, extending it to its fullest, (carefully) grasping the little rebound jobber with some pliers, and all for nought. Guessing it’s that tap I gave it that’s caused it to bite onto the aluminium threads a touch.

Any more ideas?

Sorted it! Just encouraged the exposed internal a little further out by gently levering a couple of flat blade screwdrivers twixt the nut/crushwasher and lower leg, taking care not to damage the threads. The nut unwound a treat after that!