I've just seen this post.
I'm a member of Parkstone but no longer have a National 12. One of the member had a Numinous design as toy (there's no fleet racing for them at PYC) but it hasn't been at the club for a year or so.
Have you had any success with your quest? If not perhaps try Spinnaker SC which is where we last sailed our Twelve, there may be one or two stool there.