Issue was due to 2 problems.
The payment gateway sandbox parameters were created in ecart 6.0.4 and the checkout wizard had not been re-run, so was still getting a transaction failure.
The confirm_cart.php page was checking for CheckoutWizard_submit button being posted before the WA_persistForm function was run.
I needed to check for this and the submit buttons for the other payment gateways I had added to ensure WA_persistForm was called