Seem to have it fixed now.
Seems it wasn’t .net after all – and I spent 5 hours uninstalling all versions and then reinstalling the @##@ thing!
After all that I found it was windows had included labview and PCI_something in the DEP selection.
Reverted that to the ‘windows essentials’ only and it’s ok now.
cheers for the advice anyway - at least I know .NET is sparkling clean.