Thanks for replies and links to suppliers – my Googling skills obviously need honing!
I confess that I just eyeballed the old belt and thought it looked like quarter inch, so posted 6mm. Actual measurement vary from about 4.5 to 5.8mm – it's seen a bit of work! I doubt that the exact size matters though – anything from 5 to 6.350 mm would work I expect.
Bill – no, I didn't know about the site for the manuals, so thanks for that link, but unfortunately the machine I have (201K) isn't listed.
Having done a bit more research it seems that this was a highly desirable machine in it's day, usually bought on the drip by households needing to make some extra cash in the post-depression years. £25/10 (£25.50 for any youngsters!) in 1938 translates to around £1700 today, so a significant investment for a sewing machine. What would a hobby lathe have cost at that time I wonder?
Richard S2 – thanks for your kind offer, PM on way.
Cheers, Rob.