Only seed of species clematis will come true from seed, you're better off taking cuttings, or layering them, softwood cuttings in spring, layer in late winter or early spring, Derek.

Agree with Derek for named cultivar Clematis. If you can get fresh seed of species Clematis then there is no great difficulty. Sow them on the surface of a pan of seed compost and cover with grit, then leave them outsider in the shade until they germinate.

Thank you!