I'll make sure you're able to add the TAX Registration Number to your report, spalding.

You can go to the Custom form styles panel to customise an invoice and add the TAX registration number. 

Here's how:

1. Click the Gear icon on the top menu.
2. Choose Custom Form Styles.
3. Tap Edit on the invoice.
4. Select Content, and
5. Click the Header part on the sample invoice. Here, it will allow you to add additional details into the box and a footer.
6. Press Save. 

For more details about customising invoices, estimates and sales receipts in QuickBooks Online (QBO): 

• Customise invoices, estimates, and sales receipts in QuickBooks Online 
• Add, customise, or remove logos on sales forms 
• Import custom form styles for invoices or estimates 

Keep me posted if you have further questions or concerns. I'm always here to help you.