Onedrive

[king1]

have you tried logging into https://onedrive.com website? 

In there you can see your files - bottom left shows storage used/allocated, it's clickable for more info, which also provides a link to Outlook attachments

Yes, have logged into onedrive.com. I keep going around in circles

Pic below:
Bottom left shows storage/allocation as 2.8gb used of 5gb
Clicking on it does provide a link named Outlook attachments however it merely takes me to Outlook.com which gives no means of deleting or disabling the 2.8gb backup.

Bottom right: Onedrive icon>help and setting>settings
shows 0.1gb of 5gb, no mention of the 2.8gb

Top: this is within Outlook

Could Onedrive be reporting a figure that is not actually for real??

I think when you are logging into outlook it is doing so with a different account, the two different storage amounts make that fairly clear... check which account onedrive and outlook are logged in with...