Thanks,
that has worked, up to a point. I've tried it twice, and both users get the save session option. Great.
However, user 1, Admin, had a document open in workspace 1, and firefox open in ws 2, and on logging back in document/libre was closed and firefox was still open, but now in ws 1.
I thought it might be that the document was passw protected, but I tried again with a non-passw test document, same result. But anyway it wouldn't explain why FF jumps to ws 1.
All very mysterious!