Boot the computer to safemode and run the Disk defrag - tap F8 at the dell logo screen to boot to safemode.

or you can also try  JkDefrag 3.34 - download link as below:

http://filehippo.com/download/1b0667dd13013b39d9178687be613650/download/

Tried this, but still left with 2 fragmented files ~temp_fill_file_5epEKY & ~file_file_1sIEcW both are 16Gb.

Do you know what these are and how can I get rid of them.

Are you able to move the files to a different location also try to rename and erase the file.