Thanks for chiming in on this thread, simonjude.
I'd like to add some info about your concern.
The VAT margin scheme is currently not supported within QuickBooks Online. If you want to record the transaction, I recommend reaching out with your accountant for advice. They may be able to give journal entries to help you record the adjustments.
If you're not affiliated with one, you can visit our Intuit Find-A-ProAdvisor site to find certified professionals near you.
To learn more about VAT in QuickBooks Online, please feel free to read this article: Set up and edit VAT settings, codes and rates.
For any additional questions or need help with other task in your account, please let me know in your reply. I'm always around to help you out. Take care and stay safe.