Few more clues as to what you're looking for would good - clematis vary in height from about 4-6 feet up to 35 feet, a few are evergreen, so what purpose do you want the clematis to serve? Colour in summer in a smallish area or one that will cover a fence and climb up into a tree?

That said, a Prune group 2 clematis wouild probably serve best from the long flowering point of view - they have those very large flowers for about 4-6 weeks late spring early summer, followed by more, smaller flowers about 4-6 weeks later which go on till autumn. Examples would be Bees Jubilee, The President, Niobe and there are plenty of others.

I thought all Clematis were hardy . . . ?

Depends where you live in the world, sheilabub - C. smilacifolia is described as a 'tropical' clematis, only hardy down to -7C for short periods, but generally, the range available in the UK are hardy.

Thanks Bamboo, about six feet height or a bit more, will take a look at the long flowering types you mention.?