Many thanks to everybody who replied!
I watched the webinar, and yes, it did explain things very well.
Also Jill Whitehead's article, and indeed all the others in 'Shemot' DNA edition of December 2019 (journal of JGSGB), which make a lot more sense to me now I'm actually working with the results of my own Ancestry autosomal DNA test,
--
Ric Cooper
Midhurst, UK