Hi and thanks for the quick response:
VA v1.7.5
Windows 10 Pro Insider Preview (6.2.9200.0, en-GB)
Options > Audio > Audio Output Type is "Windows Media Components"
** AHA! **
I changed it to Integrated Components and the problem goes away!
Of course, I didn't do this until I'd uninstalled and re-installed again!!!
so - looks like that setting is causing the issue - I dpon't understand what the difference is but presumably it's ok just to switch back to Integrated Components? (I hadn't deliberately changed it to Windows Media Components so assumed it was the default.
Anyway, thanks for your help, and for a great product.