I assume you mean the plunger? You should be able to get one from Chambers or Nibbs (although the ones I have had from Nibbs in the past had the wrong sized hole for the spring). Jaguar + diana 22/23 should also fit. The grub screw that holds it on is 4 ba. I usually use a bit of rod and cut a slot for a screwdriver. These can be a bugger to get out.
Yes thats right and thanks for the info - Chambers dont have them listed for the Jaguar, I'll have to go back and look at the Diana parts.