Google Checkout reports error to user but not admin

Question

Hello

I'm using the following to generate a Google Checkout:

  echo '<form method="POST"
      action="https://sandbox.google.com/checkout/api/checkout/v2/checkoutForm/Merchant/'.$id.'"
      accept-charset="utf-8">';


 while ($row = mysql_fetch_array($result)) {

  echo '<input type="hidden" name="item_name_1" value="'.$row['name'].'"/>';
  echo '<input type="hidden" name="item_description_1" value="Your chossen Subscription"/>';
  echo '<input type="hidden" name="item_price_1" value="'.$row['price'].'"/>';
  echo '<input type="hidden" name="item_currency_1" value="GBP"/>';
  echo '<input type="hidden" name="item_quantity_1" value="1"/>';
  echo '<input type="hidden" name="item_merchant_id_1" value="1"/>';
 }

echo '<input type="hidden" name="_charset_"/>';

  // Button code -->
  echo '<input type="image"
    name="Google Checkout"
    alt="Fast checkout through Google"
    src="http://sandbox.google.com/checkout/buttons/checkout.gif?merchant_id='.$id.'&amp;w=180&amp;h=46&amp;style=white&amp;variant=text&amp;loc=en_US"
    height="46"
    width="180" />';


echo '</form>';

Which I think is correct and is sending users to the sandbox checkout. However Google gives this message:

Oops! We were unable to process your request.

Time when the error happened: 2010-07-27T13:16:42 (UTC) (48c5e4fef7fe8)

But I'm not seeing anything in the integration console. I have checked and confirmed the correct merchant id for the sandbox account.

Does anyone have experience of this? Have I missed something?