You'll notice that by using double-quotes you've caused it to have mismatched quotation marks.

Try using single-quotes around your added content instead:
PHP Code:
define('EMAIL_GV_CLOSURE', "\n" . 'Sincerely,' . "\n\n" . STORE_OWNER . "\nStore Owner\n\n". '<a href="' . HTTP_SERVER . DIR_WS_CATALOG . '">'.HTTP_SERVER . DIR_WS_CATALOG . "</a>\n\n" . ' Please visit our <a href="http://www.facebook.com/GardenSeedsUK">Facebook</a> and <a href="http://twitter.com/Garden_Seeds_UK">Twitter</a> pages for all the latest offers, news and product updates');