Thanks "Rusting In Peace" !
WinDirStat called the extra missing data "<unknown>". WinDirStat help suggested this might be "System Restore Points". I googled a way to remove the "System Restore Points" and that did the trick, back to 19gb for my c drive.
Thanks for the direction, helped me get there.
For info the paging file (pagefile.sys) appears when hidden files are shown. When I do ghost backups I usually reduce this file down to minimum (in System Properties, advanced) which saves backing up an extra 4gb - and switch back to the default after backed up.