Thank you for getting back to me.
In this case the culprit was the plugin "SEO Adviser" that I installed some time ago. I never activated it so I decided to remove it to see if it was the backdoor and... bingo! Suddenly hundreds of spam pages that only a Google site search picked up on became visible in the pages menu and I was able to remove them.
What's worrying is the sophistication of the hack as neither the free versions of Wordfence nor Sucuri picked up on the script or the spam pages. :-(
Now here's to hoping that they will leave me alone for a while. Ironically, when I only had a few html pages on this website I didn't have these issues. Maybe it's time to go back to basics... ;-)
Thanks again,
Cristina