You can manually adjust the VAT on the invoice to over-write the calculated amount. As for entering a VAT only invoice, the work around courtesy of @ian_roberts here VAT only Invoices - Nil Sales seems to be to enter two lines on the purchase invoice, one for the net amount at 20% to give the correct VAT. The second line would be minus the net amount but at zero VAT. The balance on the invoice would then just be the VAT.