Hello, Siobhan Coote.

I'd also get frustrated if I can't find out what's going on with the form I'm working on. I'd like to lend a helping hand by working with you to solve the VAT amount calculation issue. 

I can see two culprits causing the calculation issue. It's likely caused by the VAT codes used or it's just a simple sync issue on the transaction form. 

Before we dive in, I'd like to ask for a screenshot regarding the line items and the VAT codes used on the bill transaction. This way, I'll be able to see what's going on and lay down the solutions to help you fix this. 

QuickBooks calculates the VAT amounts based on the tax codes and the VAT amounts (inclusive or exclusive) on a certain transaction form. 

Make sure select the correct VAT code for the import line item and the negative line item transaction under the VAT column.

You'll also want to make sure to determine whether the amounts are Exclusive of Tax or Inclusive of Tax under the Amounts are drop-down menu. 

If the calculations still don't display the correct amounts, then this might just be a simple sync issue. Close and re-open the bill transaction form to refresh the page. 

If you need to handle your returns and other VAT-related tasks, you can check out this article for a guide: VAT Feature Update FAQ.

Need to check your expense and income transactions using reports? We have an article to help you see what reports you can run: Run reports in QuickBooks Online.

I'm still looking forward to your reply regarding the screenshot. If you have other concerns regarding your taxes or transactions, I'm all ears and ready to help.