The customer details wont load when I try to edit an invoice that already has their details recorded. Is this happening to anyone else?

Welcome to the Community, multibiz-bigpond. 

Customer details will automatically populated when you create an invoice. We can perform some troubleshooting to see if this is a browser-related cause.

I'd like to start with opening a Private Window or an Incognito Window in your web browser. This will open your account in a clean slate and loads fresh data. 

Here are keyboard shortcuts you can use depending on the browser you're using:

• Google Chrome: press Ctrl + Shift + N  
• Mozilla Firefox: press Ctrl + Shift + P
• Internet Explorer: press Ctrl + Shift + P
• Safari: press Command + Shift + N

If it works, you can go back to your original browser and clear the cache. Alternatively, you can use other compatible browsers.

Please feel free to comment below if you have any other questions.