Thanks Anne, the URL's are kind of working to switch layouts, it's not 100% though?

For example the site switches from mobile to desktop when the "view desktop" link is clicked but the mobile menus are still rendered instead of the desktop ones, the left column is missing as per the mobile version and the center column that displays the top level categories is still 1 product per row instead of 3 per row (as per the desktop version)

I've added a new check for the 'isDesktop' to alter the viewport so that a user can zoom etc
Code:
<?php
if ($_SESSION['display_mode']=='isDesktop') { ?>
<meta name="viewport" content="width=device-width, initial-scale=1, user-scalable=yes" />
<?php } else {?>

<meta name="viewport" content="width=device-width, initial-scale=1.0, maximum-scale=1.0, minimum-scale=1.0, user-scalable=no" />
<?php } ?>
which is working fine?

Ive seen the mobile detect code towards the bottom of the html_header.php file which looks to control the loading of the stylesheets etc and tried tweaking it but nothing seems to fix it properly?

Thanks

Tony