I wonder if that's a genuine Pro Fab swingarm? My Pro Fab HL had a different swingarm - axle plate kicked up and the shock mounts were about in the middle not at the end. maybe they changed the design in later runs? Or maybe that's a very early version? Certainly the ProTec 600 used the same style arm as mine did.
I think the different swingarm was an option, the arm on your bike Graeme was probably the most common, it's the same as the one Aberg is on in the Profab brochure but the arm on the UK bike looks very much like the arm sitting loose in the Profab brochure.
The frame on the UK bike has all the indications of being a Profab frame
I would think a Profab frame in the UK is probably rare as would a NVT frame be in the States