Ok people I've uploaded version 2.0 to Zen Cart -- PLEASE update your websites asap.

These are some but not all of whats changed:

AES encoding added - alot stronger and harder to crack.
Error messages now display to the customer when they are returned to your website from SagePay.
Transaction details from SagePay are now recorded in the orders

Enjoy & a massive thank you to all of you who've helped & made suggestions to improve the module.