Thank you for coming back to the QuickBooks Community, angusk572. I appreciate you for performing some troubleshooting steps to fix error #12007 when updating QuickBooks Desktop. I'll ensure you'll be routed to the right support team to assist you further with this matter.

Sometimes, QBDT is unable to successfully connect to the internet, which causes a problem when updating. This may be because there are incorrect browser and security settings in place or an application such as an antivirus or firewall program is interfering with the update. 

Since you've already completed the steps outlined in the article to resolve the issue, I recommend contacting our QuickBooks Support Team. They have the tools to pull up your account in a secure environment which we're unable to do it here in the Community. This way, they'll be able to investigate the cause of the problem.

I'm adding this article so you can view different details on how to use the Verify and Rebuild features in QBDT to identify data issues within a company file: Verify and Rebuild Data in QuickBooks Desktop.

I'm just a post away if you need further assistance or encounter any issues while updating your QBDT. I'll be glad to help you anytime. Have a good one!