I am starting to build my patio and want to dot some plants within it as it is quite large. I would love to plant a Clematis over a large rock in a squared off area covered with chippings. I have several clematis is mind as I would like a strong grower and perhaps one that blooms twice a year. My collection at the moment that I can use are,Sunset, Henryi, Piliu, Nelly Moser or the President. There might be some better ones but you would you recommend either of the above.
The clematis you mention are good, also the old reliable Jackmanii, and prehaps use some of the other Viticella types, such as Chermesina, as they are good later in the summer.
They will need to be pruned carefully so as not to just make a mound.