Thanks everyone, I'll go off and do the research now. If I make any startling discoveries in coming weeks I'll report back in case it helps others attempting SSO.
*GK* P.S. I did try Auth0 about 4 years ago, but it was so over-featured and geared towards "APIs" that I couldn't identify the possibly useful bits of it. And the documentation was so full of jargon and assumptions that it would have quicker to learn advanced homological algebra. >
