Thanks for your help - that did the trick.
I have another question - We don't send users to the product description view although we ARE using the product description view to enable likes on facebook.
What I would really like to do, is if someone clicks a link from facebook, they will land on the product detail page - what I want to do is redirect them to the parent category instead.
I understand that each product can have multiple categories but is there a shortcode that returns these categories?
1) I can modify the php file to check the referred field and not perform this redirect for facebook robot
2) On the category page, I will know which product the redirect came from and then scroll the user to that product and highlight it.
Any help appreciated.