Thanks for the pointers - I've solved the problem 
There is already an override Products Details Page in the location you suggested. I located this code :
if (!VmConfig::get('use_as_catalog',0)) { ?>
and changed to
if (!VmConfig::get('use_as_catalog',0) && $this->product->product_in_stock<>'2001' ) { ?>
Setting the 'In Stock' level for a product to 2001 hides the 'Add to Cart' button for that item 
I used the above code for the product details page. It worked. I am trying to accomplish the same thing for the category page.
I've got the add to card button on showing for each product on the category page but I want it to only show under the same circumstances as above.
I'm not a php developer and really have no idea what i'm doing. Can someone help me out please.
Thank You,