Is it always semi-double or do you get fully double blooms too?
is that color how it opens or is it faded from the sun?
Do you have any other pictures?

It could be Belle of Woking that hasn't fully doubled yet.

That's exactly what I was going to suggest :-) It looks very much like my Belle after she has produced her first flush of fully double flowers.....the second flush flowers are very similar to what's in the photo. And the color is dead-on!!

This is the only flower! This clematis just turned up-- probably I planted it a long while ago and it died back. I think the color is the same as when it opened-- it is in a rather shaded area.