Originally Posted by Charlie Pridham

"
If you don't mind losing the flowers do it now, cut back hard, move the
plant with a decent root ball and water in well (don't let it dry out during
the rest of this summer)
should go away fine.

--
Charlie, gardening in Cornwall.
http://www.roselandhouse.co.uk
Holders of National Plant Collection of Clematis viticella (cvs)