Just an update,
I had no luck freeing it up with the forks on the bike so I took them off and stripped them down.
With the rebound damper out I applied some WD40 to both ends of the adjuster barrel and left it to soak, no movement.
I put the adjuster knob back into the barrel and gave it a couple of sharp taps with a mallet, still no movement.
Ended up figuring it’s knackered anyway so a quick dose of heat from the hot air stripper won’t do any harm (on the low heat setting), lo and behold it freed it all up!
So if anyone else out there is struggling hopefully this may help before you shell out for a new assembly.