Incorrect Tax Calculation with discount and VAT and tax-included setting

Just writing my own module to export multiple invoices to an XL spreadsheet, combined with a XL Add-In that exports them all to a single Word document, so I can print the invoices for the last 100 orders (for example) with a few button clicks.

Anyway, while doing this I noticed the following issue, I have an order which the standard zen invoice gives:

Sub-Total: £14.99
Standard Shipping (): £3.95
Discount Coupons: TESTCODE : -£1.50
Total: £17.44
Includes VAT @ 17.5%: £2.82

The problem here is that the VAT reported is actually the VAT before the discount was applied and is therefore not the correct amount of VAT that the customer has paid.