OK, I’m stuck.
If I start a slideshow from within my Python script, then enter a checking loop to see if it is still running, then the value of xbmc.getCondVisibility('Slideshow.IsActive') is always 1 - even if I subsequently stop the slideshow from within Kodi using the remote control.
Form this and other testing, it appears (to me anyway) that the value of Slideshow.IsActive gets set to 1 when a slideshow is started, but doesn’t get reset to 0 when it is terminated. I am a novice at this, so I may be wrong. Sounds like I now need input from the Kodi team…