Bumping this up hoping someone will see this and possibly have a suggestion for you. :)

I like the Lauresterne. Not sure on the spelling on this, but I've got one and I love it. Do a search for it, and see if you like it.
Leslie

It is really hard to answer this question. There are literally hundreds of lavender Clematis. I'd say it is the matter of personal preference. Maybe, you should narrow down your choice to several varieties and ask about their performance here? You can use the Clematis on the web search engine for example. Just enter 'lavender' or 'blue' into 'Tepal colour' box and submit.