Thanks for the quick response
Having logged on to the home page of Google+, under "Share what's New", I selected "Photos".
Then on the next page I selected "Your Google+ Photos"
This gave me a page containing thumbnails of all my pictures and videos.
I was able to copy these to my PC, but the pictures were only 9kBytes and the videos, a jerky 450Kbytes.
Do they exist anywhere in their original full size and if so, can I then transfer them.
I note that the original material on my Nexus 7 in the "Auto Backup" folder IS full size and the videos perfectly watchable.