I fixed it on mine, it was to do with the BELLMIN' Time Machine Queue Backup files (i.e. files that meant to go onto the Time Machine but haven't been moved yet due to the TM not reachable or away from home etc).
Do the following (no need to buy or download any additional tools):
Go to Terminal
List all TM objects by doing the following
sudo tmutil listlocalsnapshots /
It will come back with few items:
Remove these by doing:
sudo tmutil deletelocalsnapshots 2018-04-01-122047
sudo tmutil deletelocalsnapshots 2018-04-01-183626
Where the date is taken from the output from step-3 above.
Leave it for a while to catch it's breath. You can check the disk space by executing the
df -m command and notice the % of your Free Space, doing more
df -m every few seconds will show an increase, once done it would stabilise.