OK so thanks to all for the advice, Ive narrowed it down to the piston type from chester or Ebay number 290448499618.

But I have also seen this from RDG, who I have dealt wit in the past and found them very good.http://cgi.ebay.co.uk/ws/eBayISAPI.d...MakeTrack=true

This uses a cam system to lock and looks like the expensive Dickson ones http://www.lathes.co.uk/latheparts/page13.html

it also the looks like it would also fit on the compound slide which apparently is the easiest way to set up for screw cutting. (after hacking through the hard stuff)

Any one have a preference or experience with either type?