@SD - thanks for that. I found your post useful and am making some progress. I had already toggled 'Show hidden files' to ON, but still had 'Hide operating system files' ON as well. Now that this is OFF I can find 45GB of 'stuff' in files/folder more that 1GB in size (including 33GB in 'users' - this is music and piccies; 3GB in 'SWTOOLS'; 2GB in 'Program Files'; 3.3GB in 'pagefile.sys'; 3GB in 'hiberfil.sys) and there is probably another 3 or 4 GB in smaller files/folders. This probably totals about 50GB.
So, if I can account for (say) 50GB of data/stuff, disk says only 65GB free = 115GB accounted for. "THEY" are still hiding 30 to 40GB of my HDD somwhere.
(Somehow between earlier this morning when it showed only 25GB free and now, showing 65GB free, something happened. I pressed my blue Thinkvantage Button (shortcut to Lenovo Care Quick Links) and it showed that 'No Backup Found' so I initiated a backup. After 2 minutes error message appeared saying that there was inadequate disk space. I cancelled the backup, and suddenly the disk free space jumped from the previous 25GB free to 65GB free. This is what leads me to think that the disk-hog is something to do with backups)
Thanks agin for the help so far. Any further assistance is appreciated.