This will need hand coding to accomplish.
Probably the best way to accomplish it will be to to add a notice to the cart page that orders over £1000 get the free item. then only add it to the cart after they click the Confirm order button on the confirm page.
in other words only add to the cart once they checkout out.
The reason for this is that it will be difficult to remove the item form the cart if they get up to £1000 then remove items to get the cart below £1000.
To do this, on the confirm page, add an add to cart server behavior, set the price of the item to 0 and the name to Free - <name of the Item>. Set the trigger to Button Confirm Order Pressed.
The add to cart code will be added to the page with the following if statement:
<?php
if (isset($_POST["Submit_order"])) {
<add to cart code>
}
?>
You will need to add another if statement around that to check the cart subtotal:
<?php if($WA_Store_Cart->GetTaxableTotal() => "1000") { ?>
<?php
if (isset($_POST["Submit_order"])) {
<add to cart code>
}
?>
<?php } ?>
so that the item will only be added if they place the order and the order is equal to or greater than £1000
