It's great to see you here, @james70,
If the system is calculating a different amount for your taxes, you can edit the tax rate or add a new tax item in QuickBooks Online. Here's how:
You can delete and recreate the invoice then apply the correct rate.
For past invoices, you can adjust your VAT Liability by following the steps below:
I'm enclosing this article to help you manage your VAT in QBO: VAT: Getting Started
If you have any questions about this or anything else in QuickBooks, let me know in the comment. I'll be right here to help any time. Have a lovely week!
On the Enter Bill window, change to "Amount Exclusive of VAT" and enter the net figure from the purchase invoice. VAT will be calculated by QBs and if doesn't agree with the supplier bill (normally out by a penny or two), you can manually edit the VAT amount. The total should now agree to the purchase invoice.
However, be aware that if you should re-open the invoice later to make changes (any edits) and "Save" the bill, QBs will recalculate the VAT and you will need to manually alter the VAT again.