Forget about recording the VAT until you receive the VAT invoice (that's the document that says what VAT you have or have not paid).
VAT Invoice will show £15,900 plus £3,180 of VAT = £19,080
At the moment, you've simply made a downpayment of £4,800 on that £19,080 - so just record it as that (a payment/cheque to a supplier, Category = Accounts Payable or Creditors & NO VAT).
The deposit should now show as a credit amount against the supplier.
When you receive the invoice, enter it before entering the final payment & use the credit amount.
Alternatively (probably simpler), enter a dummy invoice from the supplier then enter the payment as you would a normal bill payment.
Once you receive the actual invoice, you can edit the dummy as required.
