SagePay Form addon not Displaying payment error messages

Hi all,
I am using a SagePay form payment module but am not getting an error message displayed when there's a problem with the transaction.
Is there some setting somewhere that I need to change to show error messages when the user is returned to the checkout_payment screen?
All I see is a 'crypt' string in the URL.
I've tried a bit of debugging with FirePHP and put some logging points in the get_error() subroutine, which if I understand it correctly should fire automatically, but it doesn't.
I tried the same thing using the built in PayPal module but when I hit cancel rather than completing the transaction, I returned to the checkout_shipping (not checkout_payment) screen with no error message, just more variables in the URL.

Am I missing something or should Zen Cart be showing errors in a red box at the top of the screen like it does in other sections?

Errors need to be set to the $messageStack if you want them to display "in a red box at the top of the screen like it does in other sections". If your addon hasn't been written to do that, then of course the messages won't display. Also, if your template has had the messageStack output section removed from it, then of course no messages can be displayed at all.