Yeah, I've done it.. but like most things it's lost in the cobwebs of time. I do remember that you can over fill the cartridges and they will lock up. I'll see if I can find it in the archives.
Edit: Found it in my journal, and all I said was
Overhauled the Strada fork dampers. Piece of cake
so, it must have not been hard. I'd left my camera at the kid's at the time, so the only picture I have is taking the top off.. you've already figured that out. It *seems like* the cartridge just unscrewed, though. Are there a couple of holes in the top for a spanner, or was that another project?