after a couple of calls to support I've solved this myself.

The issue was that when you click the 'take me there' button to resolve, there's a popup with a 'resubmit VAT' button, which changes the web page address, but the web page doesn't visibly load as the pop-up stays in place blocking the requested page. When you clear the pop-up all you get is a blank page.

The workaround is to copy the address when it changes on clicking the 'resubmit VAT' button, clear down the pop-up, paste the web address into the address bar and it will pull up the 'resubmit' page.

To solution is for someone to look at the code behind the 'resubmit VAT' button and correct it...