Thanks for the quick reply, @Muz123.

Since the Purchase Order (PO) is taxable, there isn't a way to remove the VAT Registration No. in the header of the PO. As a workaround, you have an option to edit the VAT Registration No. by going to the tax setting. Before doing that, make sure to take note of the original No. This way, you can take a look if in case you need to re-show the number in the PO or business needs. 

Here's how: 

1. Click Taxes on the left side. 
2. In the Edit VAT drop-down arrow, choose Edit Settings. 
3. In the VAT Registration box, put an asterisk (*), then hit Save. 

For more information about the process, please browse through this article: Set up and edit VAT settings, VAT codes, and VAT rates.

Please feel free to read this handy article for your reference. This link contains steps on how to adjust VAT liability, accounting method, and many more. 

• VAT: Getting Started

Let me know if you have follow-up questions. I'm always here in the Community ready to help, take good care always.