The quick and dirty way of doing it would be to make index.php simply
Code:
<?
readfile('http://www.shopscifi.co.uk/catalog/index.php');
?>
though you would have to check all the paths on the catalog/index.php page include /catalog/ otherwise the images or links won't work correctly.
The other option would be to make index.php
which would redirect the user to the location page. That will save you from any path issues and I don't think having catalog in the url is really a big issue.