I don't really know, Phaidra. B3 said it had been ID'd for her. I googled it too and I could only find it in deep red. I hasten to add, mine is a much deeper purple than appears in the photograph. Perhaps the very strong light altered the result.
Perhaps I should take a better photograph and ask for identification help.
If It helps my photo was ,a bit over exposed and the clematis was a darker, richer colour in reality.
I would take another picture but the hot sun the last few days has faded them so they look more like my photo now anyway.
I don't think that there have to be identical numbers of petals on the same plant.
Richard Hodson will know , if he looks in.
Well, here is a new photograph, taken at 7 this morning, before the sun hit it. Its colour is true to life and, as Punkdock says, the petals vary in number.
Some of the petals look chewed but they're not; they're simply curling backwards.
I will put in for an ID, just in case someone can help.