Cross referencing payments shouldn't be difficult, so when clients dont use the correct (or any) invoice references, the only other data you can go by is the invoice amount.

Payments hit the bank account as gross payment, invoices only show net amount, so one has to take the net amount and add the 20% VAT to get the full figure for cross reference.

Adding a Gross Amount would fix this. (Unless there's another way to see this that I am missing?)