"Windows Defrag had done most of the work but for some reason had failed to finish. Does anyone know why?"
Yes, you said: "I ran WDD but after 18 hours it had still not finished! I therefore decided to cancel the defrag"
IMHO you should have at least 15% of free drive space for a defrag to run in. There is lots of file fragments swapping to be done :)
If you'd done the cleanup - empty temp files and recycle bin etc before you ran the defrag that would be some of the GBs reclaimed and then if your harddrive is very big and very fragmented it would do away with the numerous 'file fragment pointers/indexers' kbs and the 'empty' part of the blocks in each fragment (think of them as a box, you put part of a jigsaw in one box and part in another box etc, then pile the boxes up and it takes alot of space. Now put all the parts of the jigsaw into one box and - voilà you have lots of empty boxes available to pack something else in) and it would take a long time shuffling all that about.
I agree that you get more detail of what's happening in a defrag using the Auslogics version as MS took away the detail in Vista version. (They've put it back in again for W7 upwards)