Thank you for your thourough reply.
The first 2/3rd of your reply was of little help. Rebuilding spotlight didn't work and all of the photo trashes etc was already emptied.
That left me with running OmniDiskSweeper as a super user. I think we are getting somewhere and I have discovered the missing space.
I see a folder called .MobileBackups.trash which is close to 90GB in size which just so happens to be the space that I am missing.
In the Storage display of System Information this folder is being listed as Other and not backups. It also doesnt appear to be deleted when space is needed. Can I safely delete .MobileBackups.trash?