Thanks Earnie that VideoGuard info was the last piece of the jigsaw for me. Everything is working fine now. I watched the whole Grand Prix (2 hours) with no breaks in service.
So just to recap for anyone tuning in who is using the Sky Sports App with Virgin Media log in on a Windows 10 laptop.
Check that you have the latest version* of Sky Sports App (currently 126.96.36.199) and the latest version of Cisco VideoGuard Player (currently 188.8.131.5252) and most importantly make sure that you have uninstalled any old versions of Cisco VideoGuard.
*check installed versions via Control Panel/Programs and Features