Raven, have a look at above link, and you can see the spacers....
Mr M, They're adjustable by removing the inner bolt and then turning the eye end. Bit fiddly on the aligner, but once they're done, they're set for good. Myself and the local Pedders boys here (gratuitous sponser plug) have got it down to a fine art. :hmm:
On 20's, putting more thread on the castor arms helps things, too.