(in fact my discount percentage is very slightly too high, so the subtotal is £748 instead of £750, but the calculation order is still wrong. The system is calculating the VAT on the full price not the discounted price).
Why is this occurring and is there an easy fix here?
This is occurring because on your site it seems the Promotions Affect Taxes is set to No. You can fix this by going to Event Espresso > Promotions > Settings then select Yes next to Promotions Affect Taxes. Then click Save.