That's great. Thank you.
Have added the code as instructed and it works just fine **but** in order for the new shipping rate to be calculated correctly on confirm.php, I have to refresh the page before the additional amount is added.
1 - View Cart
2 - Checkout and add address details with an 'IM' post code.
3 - confirm.php shows standard shipping rate
4 - Refresh the confirm.php page
5 - confirm.php now shows the correct shipping rate.
Any ideas how I can make the page show the correct rate on it's first load?