Thanks for getting back, I was out and about, back now.
This looks cool. *.(txt|mp4|jpg|iso)
Before I posted I thought of that as an explanation but then I observed the checkbox for "delete destination files that aren't in the source" and so I posted.
So is that redundant? If so should it be greyed out?
If this item is not in use "delete destination files that aren't in the source" can the equivalent be added to say *.(txt|mp4|jpg|iso)
Turn off 'edit as text' and just put the more complex pattern into the first of the Name Match clauses shown in your original screenshot, and then delete the other clauses.
I guess I'm lost at this stage because that's the very thing that doesn't happen, ie It doesn't delete files "that aren't in the source that do match the filter."
However, I'm very grateful for the prompt and helpful response plus the new (to me) way of doing it.