To elaborate on what Marcus said -
1.Loosen the ring nuts that adjust the spring tension all the way loose so the spring flops about.
2. At the end with the rebound adjuster, separate the spring collar at the shoulder. Do this by holding the bottom out bumper and whacking that part of the collar that touchs the spring back towards the spring - if it is dirty it will appear to be one piece, it's not. I use a plastic hammer and whack around the edge evenly all around.
Once these pieces are separated, remove the wire circlip and then the collar can be removed.
See
HERE for a picture. The wire ring is 21, the collar 31 separates from the piece with no number between the bottom out bumper 10 and shaft(rod) 9.
I hope that's clear.
Kiwi