Okay this is now resolved, it is fixed by using symlinks on linux.
Insted of making a copy of index.php and .htaccess to a subfolder in your magento directory.
Simply do this…
Okay you Magento in installed in…
and domain magento.mycompany.com points to that folder.
sellingwebsite.co.uk points to…
This is the folder you want to copy your index.php and .htaccess files to.
Edit index.php to say Mage::run(’sellingwebsite’, ‘website’);
The line $mageFilename = ‘app/Mage.php’; should stay the same.
In the /var/www/vhosts/sellingwebsite.co.uk/public_html/ you will need to create some symlinks.
ln -s /var/www/vhosts/magento/public_html/app/ /var/www/vhosts/sellingwebsite.co.uk/public_html/
ln -s /var/www/vhosts/magento/public_html/js/ /var/www/vhosts/sellingwebsite.co.uk/public_html/
ln -s /var/www/vhosts/magento/public_html/media/ /var/www/vhosts/sellingwebsite.co.uk/public_html/
ln -s /var/www/vhosts/magento/public_html/skin/ /var/www/vhosts/sellingwebsite.co.uk/public_html/
ln -s /var/www/vhosts/magento/public_html/var/ /var/www/vhosts/sellingwebsite.co.uk/public_html/
Point your sellingwebsite.co.uk domain to /var/www/vhosts/sellingwebsite.co.uk/public_html/
In the sellingwebsite config set the domain as the base url insted of base link url as said in the video.
You should be good to go.