Finally got hold of a strap wrench to undo the top collar on the post.
Cleaned it out (not that there was any grease or gunk) and applied some fresh SRAM Butter below the bushing.
From all the way down it goes up completely with a satisfying thud when it stops at the top.
However, when it is only compressed halfway down the travel it refuses to pop up and is also had to press down.
Fun one huh?