Brian Collingwood has an excellent site that discusses growing clematis from seed in detail.....it is pretty easy to do and there are several forum regulars that produce quantities of seed-grown clems each year. Just so you are aware, seeds from named cultivars of hybrid clematis - like the Duchess of Edinburg - do not necessarily germinate true to the parent plant and therefore cannot be called by that cultivar name. That can only be done through asexual propagation - cuttings, layering or division. But you can certainly grow some stunning vines by this method.

Here is a link that might be useful: clematis from seed