Hello, @HALCYON.

I understand the impact this has on your business and we're here to sort this out. 

Since you're already near your VAT return deadline, you can use the QuickBooks Desktop trial version and open your company file from there. Be sure to create an extra backup of your file so you can still open it on the older version. 

You can check this article on how to download the trial: Download a trial of QuickBooks Desktop.

Once done, you can use the backup to restore your company file on the latest QuickBooks version. Just refer to this guide for the steps: Restore a backup of your company file.

The validation code you have may not work if you've already exceeded the number of times QuickBooks was installed. If this is the case, I'd recommend reaching out to our Desktop Support so they can verify if the code matches your QuickBooks' license number. This way, they can provide you with a new validation code.

Know that I'm always here if you need anything else. I want to help however I can. Take care and have a good one.