Thanks for your response brummie.
I have now seen a Thorn brochure where it is described as “Lightweight, nimble, responsive, fast …..idea for Commuting, winter training, light touring or club use”. The description also says “Tight, compact frame, fillet brazed in the UK during the off peak season by our master craftsmen”, which gives the impression it was not seen as a mainstream product even when still being produced.