Why is my VAT 'paid on purchases' figure showing a negative?

I can explain why a negative amount is showing in your Purchase VAT, Kay.

When you record VAT transactions in QuickBooks Online (QBO), the amount will be posted in your VAT Control, which is a liability account. The sales VAT will post as a positive number, while the purchase VAT will post as a negative number. At the end of the quarter, the VAT filing process in QBO will post a journal from VAT Control to VAT Suspense, where the owed balance will be transferred from one liability account to the other. This liability will remain in VAT Suspense until the amount is paid to HMRC. 

Another possibility is that you might be using the accrual basis accounting method. In this case, you should select the cash basis instead, as it records income as it's received and expenses as they are paid. 

Furthermore, you can also visit this link that will showcase how you can manage your VAT returns: Pay And Manage VAT In QuickBooks.

Post your comment in the reply if you have any other concerns with VAT purchases. I'll be more than happy to assist you anytime. Have a nice day ahead!