Having decided that the remedy to a full hard drive was to migrate my photos/videos to iCloud I have finally upgraded from Mountain Lion to Yosemite.
Which went flawlessly despite the 'just a minute remaining' taking several hours.
Now comes the fun part, the upload of > 500Gb of photos and videos to iCloud. At the current rate, 30,000 images will take a few months. Should be quicker next week when I am back to work and onto a decent network.