Thanks Jason - although I'm still missing something...
Followed that with the bindings > form data > checkout.php but wasn't sure what it did.
Although it did appear in the bindings list OK.
What I'm not sure of is how to set the values of the fields in the form to the Validated Field value. I tried dragging the field across from the bindings panel, but that looked like:
<?php echo((isset($_POST["firstname"]))?$_POST["firstname"]:"") ?>
When I think it should look like:
<?php echo(ValidatedField("confirm","firstname")) ?>
Have attached a couple of screen grabs, and the page file.