Let me help you sort this out, @kstorrie.
The error prompt CLIENT_OR_AGENT_NOT_AUTHORISED means that the credentials used to connect QBO to HMRC for MTD are not authorized to retrieve details or do a VAT filing for the VAT Registration number in the VAT settings.
There are several potential reasons you’re prompted with this error.
To resolve the error, here's how:
You can refer to this article for more detailed steps: We can’t connect to HMRC right now.
Keep me posted if you have other questions. I'm always here to help.
Thank you for your help :) added notes to the below to try and identify where the problem is,
We would be grateful if you can disconnect from MTD/HMRC and reconnect to establish if this resolves the issue.
Select the cog/gear icon in the top right hand corner of the screen, once logged in, > account and settings > advanced > within the mtd section select learn how to disconnect, once disconnected select learn how to connect in the same section and follow the instructions.
Thank you - disconnected twice and re-granted access but no success, will call HMRC tomorrow to see if anything wrong from their side with my account - may mean a late submission :(
I have sent them the money at least - just unable to connect to send them the detail...