I'm glad to know that you already got your VAT number from HMRC, @fdcarroll.
Adding your VAT number in the invoice is just a few clicks. And I'm happy to walk you through the detailed steps to accomplish this in your QuickBooks Online account.
Here's how:
Go to the Taxes tab on the left side.
Click Edit VAT drop-down and choose Edit settings.
Enter the number in the VAT registration box.
Hit Save.
Once done, the VAT number will auto-populate on your sales forms. You can create a sample invoice to see what it looks like. Refer to the screenshot below for your reference.
Please visit the links below to learn how VAT is calculated in your invoices, configure tax codes, schemes, and other related issues: