Watched the video tutorial "how to change an invoice no" but when I follow the instructions, invoice number box isn't even there. just an empty space where it should be?!

Hello there, @michael-pocock, 

I appreciate you watching the video tutorial about changing an invoice number. 

Since the Invoice no. box isn't showing on your end, it's possible the Customer transaction numbers wasn't turned on in the Settings. To help fix this, let's make sure it was turned on by performing the steps below: 

1. Go to the Gear icon at the upper right. 
2. Under Your Company, select Account and Settings.
3. Choose Sales from the left menu. 
4. Under Sales form content, check the Custom transaction numbers box.
5. Select Save.
6. Click Done.

After that, let's create an invoice again to check if the Invoice no. box is now showing. 

For more details, you can check out this helpful article: Change an Invoice Number. 

The steps and screenshots above will help update your invoice number, @michael-pocock. I'm here anytime you have other concerns. Have a wonderful day ahead!