Is there a way to create a report showing the VAT codes used on invoices, please?

Welcome to the Community, @Working_again.

Yes, there is! You can pull up the Sales by Customer Detail report and customize it to show all the invoices with the VAT code used. Let me show you how:

1. Let's go to the Reports menu at the left pane and enter Sales by Customer Detail on the search field.
2. Now, click the Customise option locate on the upper right corner.

3. On the Rows/Column section, put a checkmark to VAT Code.

4. Now go to the Filter section.
5. Put a checkmark in the Transaction Type and choose Invoice on the drop-down.
6. Hit Run report to refresh.

You can also choose your preferred date on the Report period drop-down.

For additional information about customising reports in QuickBooks Online (QBO), check out this article: How to customise reports.

Please let me know if you have any other questions or concerns. I'm always happy to answer them. Have a great day!