Also I've been doing a little googling around looking at catalogues and frame numbering.
The X prefixed Frame Number isn't really recorded anywhere. The position and size of the number looks similar to raleigh or carlton but they don't use an X.
the only reference to X prefixes I've found is this from classic lightweights Hobbs of Barbican pages:
"Post-war Hobbs, like most other manufacturing companies, were required to export as a pre-requirement to being allowed materials to sell on the home market. Export frames appear to have frame numbers pre-fixed with the letter X. Thus an all chrome Blue Riband road frame, which has spent most of its life in the Rockies, is numbered X3005."

This doesn't help that much, I guess if it does mean 'export' the X could be applied to many manufacturers and the example they give is 4 digits where this frame has 5.
Dawes seem to have lost all their records of pre 90's frame numbers so maybe thats a possibility ?
You see loads of bikes from the 70's and 80's with those Prugnat 62d style lugs online too, so that seems like a dead end unless maybe they are marked under the paint (looks like a respray especially because the headset doesn't match) which could tell us if they are copies or original ?

Looking forward to getting the photos and measurements out there, thanks again guys.