Fatal error in cart if going back to Billing Details

Hi there,

In my cart, if I use it normally it all works fine but if I press modify next to Billing Details to return to it I get the following error message:

Fatal error: Class 'Controller' not found in /var/sites/s/sconchtest.co.uk/public_html/catalog/controller/common/seo_url.php on line 2

Here's a screenshot showing the issue as it appears on my site:
Please could someone help me find what is causing this error and how to fix it? I cannot always persuade it to happen either so not sure what would cause an intermittent error?!

Thanks,

Hugh

not very sure, but if it's random, might be your web host problem