When you say "a Purdey bolting system" is it a hidden third fastener? How about ejectors?
I understand you could order anything you would pay for, and that gun sounds unique. Do you have any pictures-I'd love to see them.
I always laugh when I hear the stories about "losing" a detachable lock-Der Klunkermeister, R.I.P. once opened his sliding breech Darne over deep water, a gun that sounds like it may have needed some adjustment, as he dropped the breech into the water, never to be seen again!
If the breech slides off your Darne when you open it, it needs to be fixed. Ditto if the locks fall out of your Westley!

Best,
Ted