Thanks for the replies. I have now simply copied over the files from the contribution, as there were only two minor changes in two of my files which I can put back in once this is sorted out.
I am still getting the same thing, TEXT_NO_PRODUCTS when I view a product that has related products. I DON'T get it when I view a product without related products.
I see in family_listing.php where TEXT_NO_PRODUCTS is displayed but I don't really understand enough of the preceding code to tell what has happened to display it. Could it be a database problem? I'm stumped. :(