If you measure the mounting details of the mandrel, any chance that you could make your own on the machine, if all else fails?
Lathes UK website does not seem to list it.
The CLM500 manual seems to show the chuck as fitting direct to a flange on the mandrel, (No mention of a backplate)
So maybe chucks bolt directly onto the flange. If this is the case, you have a choice between drilling/tapping the Flange to suit the 4 jaw Chuck, or drill and tap the chuck to match the holes in the Flange.
Howard