Flocsy,
I'll go out on a limb here and speculate this is an original Sherpa (making it effectively a Mk1), rather than the Mk2 George and I have/had.
One tipoff is the glossy grey/black paint. As I recall, the Mk2 was only available in gloss red and matte black.
Also, while the dropouts pictured on this one lack the upper sockets, they also appear to have only a single eyelet for co-locating the rack mounting stay and mudguard stays on a common bolt.
Certainly, the frame features look authentic -- the fastback attachment of the seatstays to the seat lug, the lack of a clamp reinforcement band, the presence of a threaded cable stop bridge, and fork/crown look right with the proper tubing content sticker. Component selection (if original, likely so) points to this being a late Mk1/original Sherpa.
You may wish to review the Wayback Machine archive I started for original Sherpa brochures and reviews. Unfortunately, many have been removed from the server after so long and have become unavailable for download/viewing since I posted the links, but there is still some info. Note this would be for the Sherpa as distinct from the Sherpa Mk2.
Hoping this helps and remember, much is based on familiarity with my past Mk2 ownership and also on inference from the information still available. I do feel confident this is an authentic Thorn Sherpa, but most likely an "older" one. The stamped serial on the underside of the bottom bracket would tell for sure, as the format is pretty standard across the Thorn product line.
Best, Dan.
EDIT: Apologies! I forgot the link to the archive:
http://thorncyclesforum.co.uk/index.php?topic=6975.0 I'll speculate further and say based on website archives, specs and photos, the Sherpa pictured most likely dates from about 2003-2010, but the components tip me more toward late in this first-generation Sherpa product run.