Hi Barberlives,
Thanks for your response. Some of that is useful - I have reduced the number of aerial themes shown by hiding Cityscape.
However I believe you have mistaken my issue. In General >> Screen Saver >> Type I have removed Aerial and selected Apple Photos >> Nature. Aerial is NOT selected. My problem is that within a few days, my choice of Apple Photos will be changed and Aerial will once again be selected without my having changed it.
Anyone know why Aerial is always reset despite setting to Apple Photos instead?