Zen Cart 1.3.8a
Google Checkout Module Version v1.4.7

I am new to Zen Cart and have installed the software and it seems to be running well.
I have installed Google-checkout and have hit a snag.

As I step through the payment process, The shopping Cart page shows the basket correctly and the button "goto Checkout" is available. When clicked it takes me to step 2 "Payment Method"

The details of the Billing address and the order total is correct and the "Continue Checkout" button is available.

When the "Continue Checkout" is clicked it takes me to Step 3 - Order Confirmation. All the details on the page are correct and the button "Confirm the Order" is available.

When the "Confirm the Order" is clicked the page reverts back to the 1st "The Shopping Cart" Page.

Checkout settings as follows:-
In the google-checkout module the enable option is set to true.
The product and merchant Keys are correct (c & p for the google screens)
Select Mode of Operation
htts://checkout.google.com/
.htaccess Basic Authentication Mode -True ***

*** Not sure how this is set up, either automatically or by hand, or how to check that it is correct.

If anyone could help me to get this working I would appreciate it as I really do not want to have paypal as the payment module.

Thanks in advance

Tim