I go to add payment. The invoice says £750 but when you press add payment it says £0???

Hello there, @jonandleo.

I'm here to help you record your customer's payment successfully in QuickBooks Online.

When receiving invoice payments, make sure that you've selected the correct invoice. This way, the amount will show up on the Payment section. If the the payment field is empty and the open balance is showing the invoice amount, you may manually enter the payment.

To do that:

1. Select + New.
2. Click on Receive Payment.
3. From the Customer dropdown, select the name of the customer.
4. From the Payment method dropdown, select the payment method.
5. From the Deposit to dropdown, select the account you put the payment into. 
6. In the Outstanding Transactions section, select the checkbox for the invoice you're recording the payment for and enter the amount if it's zero.
7. Enter the Reference no. and Memo if needed. This is optional.
8. When you're done, select Save and close.

To help you in managing customer's payments in QBO, please see this link: Record invoice payments in QuickBooks Online.

For your future reference, you can always use this article to guide you in recording refunds in the system: Learn how to refund a customer or supplier in QuickBooks Online.

Should you need additional assistance about this or with QuickBooks, please let me know. I'll be around for you. Take care.