I read on an American-based Seca 11 site that a shock from a V-Star would fit a 600 Divvy. V-Stars seem to be pretty rare in the UK - I think they were called Dragstars over here?
I found this Dragstar shock advertised on E-bay (item number 140904755965):
I don't actually need one at the moment, but as Simon said in his post 'someone always is'. I thought it would be useful to identify a source of second hand shocks, as an alternative to Hagon etc.
Price for Dragstar shocks seems to be higher than Divvy shocks, but they also look to be younger and better condition.