Cheers for the advice. I just pulled the damper out a couple of times, wiped down to remove a small amount of oil and reinstalled. I can now get about four inches of travel just bouncing on the fork in the garden. Should mean I will get full travel if I hit something really hard, so about right, then.
I did notice that the piece that opens and closes the aperture at the bottom of the damper (when the adjuster is turned) waggles a little bit independently of the adjuster. I have made sure it was fully open when it went back in. I have also not bothered to put the adjuster cap back on. I never lock the fork anyway, and don’t want to close the damper via the adjuster then not be able to reopen it fully without having to get the damper out.
I might see if motion control dampers can be bought separately.