UPDATE to my previous post I retried one more time as follows;
I uninstalled Kaspersky, restarted my computer, then retried Unreal, retried importing via Datasmith (worked fine as expected since Kaspersky was uinstalled).
THEN SAVED the Unreal Project (this I didn’t try the first time).
Just for luck, I created a brand new project, imported again the 3ds Max scene via Datasmith button and again SAVED the project.
I then re-installed Kaspersky, restarted computer (just for luck!) and retried Unreal, importing via Datasmith, and it’s working fine 
So… SAVING your project a few times once you’ve imported via Datasmith (maybe once might well be enough I don’t know) fixed things for me.
Many thanks