Updating Magento Attribute Programmatically

I have been writing a script that updates my attribute add_ten_pence, the attribute is a yes/no boolean value. At the moment it goes through the SKU's in the database but unfortunately it's not updating the database. I have pasted the script below does anyone know where I'm going wrong?

The attribute is set to 'No' as default, and I wish to change it to 'Yes' when the script runs, and vice versa.

require_once('app/Mage.php');

umask(0);
Mage::app('default');
Mage :: app ()->setCurrentStore(Mage_Core_Model_App :: ADMIN_STORE_ID);
$productCollection = Mage::getModel('catalog/product')->getCollection();

foreach($productCollection as $_product) 
{
    echo "\n".'updating '.$_product->getSku()."...\n";
    $product = Mage::getModel('catalog/product')->load($_product->getEntityId());
    $product->setAddTenPence(true);
    $product->save();
}