Well, I’ve fixed the lockout. And this serves as a very good example (among many other on the internet) of when the forums win and the manufacturers guidance doesn’t.
The cause of my sticky lockout was friction between two parts of the lockout mechanism (where there shouldn’t be friction) that the lockout return spring is not strong enough to overcome. I simply solved it by sanding away the black anodising on the inside of the 24mm nut where the threaded lockout nut sits on top of the damper.
Here’s the video I followed, with a much better explanation than I can write. Note that my damper looks slightly different to the one in the video but the fix was exactly the same - my Rebas cartridge is white instead of red, and instead of a half moon on the bottom, mine has a blue disc with 2 holes for oil flow that are closed when the lockout is engaged.
