Sorry to say, that I don't think that Pete Snidal ever wrote a manual about the UCE engined machines, or updated his book about the IB models to include info. about the UCEs. You might be looking for a refund, not an alternative download.
Unfortunately, there's a lot of online vendors who use the word "classic" to refer to the iron barrel machines I think this was the practice before the word was adopted, upper case, in the names of the UCE "Classic 500" and "Classic 350". Plenty of research, caution, and questions asked of vendors before any purchase, is a good policy, in my opinion.
As far as a manual, I did find at least one PDF of RE's service manual for UCE machines available for download online, and printed that out and put it in a loose leaf binder. That is probably still out there.
And I also purchased the Haynes manual, which covers the UCE line, and which I found useful.
https://www.amazon.com/Enfield-Bullet-Continental-Service-Repair/dp/1785214276