Have a look at these links they give excellent advice on pruning clematis. http://www.clematisnursery.com/Pruning.aspxIt
www.gardenaction.co.uk/plantfinder/clematis_
It depends on which Clematis you have Pat12. As a rule of thumb I was told Flower before June Don't prune. Later Flowering, cut down to 12" from the ground in early spring as it flowers on new growth. If your early flowering clematis have outgrown their position then cut back unwanted and damaged stems after they have flowered. Hope this helps.

I wrote a blog to try to help on clematis pruning...click on my 'dog', go to 'blog archive' and you'll find it in June 2009. I hope it helps.

It is a bit late in the year, but as everything is so late anyway, I'm sure your clematis will thrive if it is a type that needs a prune.