What is the best Clematis to grow up a Yew
I have two large Yews in my garden which form the entry to the lower part of the garden. I would like some more colour so would like to plant a clematis on either side to grow through the Yew. Its a south facing garden, with clay soil and my colour scheme is white and some purple. If it grew in the Yew it may be tricky to prune. I'd like something which would flower twice a year. Long list I know but which would you recommend?