Well in case anyone ever gets in the same situation (too many errors to repair) I managed to resolve this today. After much frustration and trying to avoid a clean reinstall - it's always 1000% more work than I remember - I had one last try. I'd tried various ways to reinstall a new copy of Windows but they all failed. Today I tried the Media Creator and selected "Upgrade this PC" 45 minutes later and all resolved. No apps lost, few settings lost but no big deal and Windows updated to 16299.309. However SFC has just reported no integrity violations
Now just need to know why Windows Update tells me I'm up to date but 16299.334 is out there!!