I think that is what this little purple nodding clematis is; it covers this obelisk and is quite vigorous.
Clematis alpina
Does it look like any of these Clematis alpinas? http://www.clematis-info.com/wms/wmsg.php/11840.html
Yes it does, Shirley only more purple looking than the Pamela Jackson shown in your link. It has 4 petals and hangs downward like those shown. I will see if the tag is still by the plant.
