Thank you for your kind endeavors. If I go to Photos preferences (icloud) , the following are ticked:
ICloud photo library
Download originals to this Mac
My photo stream
ICloud photo sharing
In my own endeavors I played around with the button next to the iCloud Photo library marked "Pause for one day" by clicking on it, the writing fades and I do not know if syncing is then temporarily stopped for one day or not in this position. This action however stopped photos from unchecking itself as per the original problem identified. Having this "paused for one day" button faded or not faded, the iCloud library still reflects "downloading 1868 photos" with this figure not decreasing if the button is in its faded or not faded position. May be a stupid question, for downloading to occur, must this button be faded or not?