You're a genius! It worked!

The HTTrack Browser ID was the problem. I changed it to your recommendation
and HTTrack started churning away as normal! 

I was certain I had a good rudimentary grasp of the scan rules and was baffled
for days when my initial scan rules didn't appear to work.

Is it worth raising a "bug report" to make the browser ID a default setting or
more prominent in the HELP file? And of course, I've now found it half-way
down in the FAQ!