When I invoice a customer they are receiving the email with the total amount on but they can view the complete invoice as it is saying invoice unavailable?

Welcome to the Community, @Markcrowther.

There are times that unexpected behavior occurs. Clearing the cache will help resolve the issue.

You can let them try using a private browser when opening an invoice. It doesn't save your browsing history making it a great place to test browsing interruptions.

Try these keyboard shortcuts depending on the browser you're using:

• Google Chrome: press Ctrl + Shift + N  

• Mozilla Firefox: press Ctrl + Shift + P

• Safari: press Command + Shift + N

If it works fine, clear the browser's cache to remove the log files and start with a clean slate. You can also advise them to use other browsers if the same thing happens. 

Check out this article that will give you more detail on how invoicing works: Create invoices in QuickBooks Self-Employed.