Hi Geoff. You have two choices. Number one. If your bike is still under warranty then most dealers will put in a claim to replace the whole rear shock including a new knob which is supposed to have an 'O' ring fitted to help stop water and dirt ingress.
Number two. Get a piece of wood long enough to reach through the underside of the bike and then tap the knob off with the aid of a hammer onto the wood. { this is as long as you cannot physically pull it off by hand} Then once the knob is off then get a socket on the bolt which is exposed underneath, and with the extra pressure from a socket and bar or ratchet you should be able to slacken the bolt enough to expose as much thread as you can . Then lube the thread up well and give it a good working with the socket until it goes nicely again.
I believe that under the knob there are 2 little springs and balls which make the clicking sound as you adjust it. If it's too much of a fiddle to get the springs back in, then just put a permanent mark on the outside of the knob and one full turn should count as one click.
A lot of 'knobs' in that explanation
but I hope that helps.
Explained to me by blumeeni who did his some time ago. Mine was all replaced last summer after an oil leak from the suspension unit
