Thanks for the offer of help. If necessary I will try Safe Mode and/or opening another account. I have restarted several times, but the "looping" always reappears eventually - with different lengths of loop!
Answering your other questions: I am on 10.13.2, which went in without problems; the photos are on the Mac's internal 1 TB hard drive (of which 75% is still available); and I have not identified any "trigger' event or appwhich creates the "looped" set of photos. I backup the whole machine to an external 2 TB hard drive, using Time Machine, but I do not include the photos in my iCloud backup as they are too big (when I sort this problem out I will probably pay for more iCloud) - although all that is of course irrelevant to the "looping" problem.
I am also trying to see if I can create a randomised screen saver using Photos - following an idea in the support community (dialabrain and Noach on December 10) which is using iPhotos but that does not seem to work