Invoice missed off VAT return how do I amend?

I'd like to share my thoughts about your concern, Lesley,

If you've already submitted your VAT return, I'd recommend contacting HMRC and complete the VAT 652 form. Your government has specific rules when making amendments to previously filed VAT returns. Hence, the said form is not available within the program. 

Here are some tips to complete the form:

• If you've added the adjustment to your return, let HMRC know by answering YES in the box marked "Adjusted in VAT return" on form VAT 652.
• Amounts payable to you should be rounded up to the nearest pound.
• Amounts payable to HMRC should be rounded down to the nearest pound.
• Input and output tax errors should be separated and entered on individual lines.
  In the "Type reference" box indicate whether the errors are input tax or output tax. Enter "0" for output and "1" for input.

Note that you can only include the adjustments on your next VAT return if the amount don't exceed:

• £10,000
• 1% of the box 6 figure required on the VAT return for the period of discovery, subject to an upper limit of £50,000

If you haven't submitted your VAT return to HMRC, we can still review the VAT Detail Report:

1. Select Reports on the left menu and search VAT Detail Report.
2. Choose the correct date range in the Report period field.
3. Click Run Report.

After that, we can adjust your VAT liability if necessary. Adjusting VAT liability depends on the affected VAT box. Click here to see more.

Here's an additional reference when submitting VAT returns: Submit a VAT return in QuickBooks.

This should help you all throughout the process. We're always here to guide you if you need anything else.