[Derek]

The cush drive rubbers have an inner and an outer mild steel sleeve to which the rubbers are bonded. On Dan's bike the bonding has let go between the rubber and the outer sleeve allowing the rest of the bushing to slip in towards the wheel. You could try superglueing it back in place but I suspect it would let go again pretty soon.
It would probably be best to replace all the rubbers but you'd probably get away with the just the one if the others are in good shape - they are a bugger to replace I believe, I've never had the pleasure.
You'll need to find a drift which is a marginally smaller diameter than the bushings but fits comfortably on the outer sleeve, an old socket maybe. If there is a shoulder on the inside you'll need to knock them out from the back which will probably be very difficult. If there's not then it might be easier to knock them right through. Best of luck.