If the invoice shows the total net amount and the VAT based on just the discounted amount (and after early payment there is a zero rated credit note for 2.5% of the invoice), then I would use a spreadsheet to calculate all of these individually which will take the pain out of these calculations.

The amount payable in this scenario would be 0.9790795 of the total if you wanted to calculate based on the total of all invoices!!

A quick stab at the formula would be (assuming F4 equals the invoice total being total net plus VAT based on 97.5% of net amount)..........

=((F4-(19.5/119.5*F4))*0.975)+(19.5/119.5*F4)

This shows that the amount payable on the total above would be £20,817.22.........then a credit (or credits) of 444.81 would be due.

If you input the invoices into a spreadsheet one at a time then it would be a lot easier......just 97.5% of the net plus the VAT shown on the invoice!! Of course the formula above could probably be simplified!

If the invoices do not reflect the scenario above.......then please explain how they are made up. I hope I haven't misunderstood!