PayPal Set up problem

[ardhill]

Hi

I am trying to set up PayPal on my ZenCart and get this message when I click on the Checkout using PayPal button that appears on my shopping cart:

We are sorry for the inconvenience. The PayPal account authentication settings are not yet set up, or the API security information is incorrect. We are unable to complete your transaction. Please notify the store owner so they can correct this problem. (10002) 10002 Security error - Security header is not valid

Some details:
I haven't check out anything yet through the cart, it's - still under construction.
Zen Cart 1.3.8a
Database Patch Level: 1.3.8
I have just set up the Business PayPal account.
PayPal for website payments standard and express - not pro
I thought I had done all the bits right but obviously not, but have gotten my Credential API Signature details, IPN URL & return URL, Payment Data token and have ensured that Business ID field in the PayPal set up on the cart is exactly the PayPal e-mail address.

Any ideas what I have gotten wrong?

EDIT: I just put it into debug mode and have a bunch of e-mails with 'stuff' in them that don't mean much to me. Though this appears to be important "10002 Security error - Security header is not valid " whatever that means.