Clematis grow easily from seed. You will know that they are ripe when they start blowing off in the wind; there will be so many that losing a few will not be a problem! Being one of the Ranunculaceae family the seed will germinate best if it is sown as soon as it is collected rather than keeping it until spring. The problem is that Clematis 'Niobe' is a named cultivar and so the resultant plants are likely to be different to the parent' you will not know until it flowers but it is worth trying.

You'll have to be patient, though, to get any plants to flowering size.

thanks for the info, that's my day sorted tomorrow, are they best kept in the greenhouse? and covered or un covered? and what sort of soil is best?

A gritty compost and left outside in the shade. Sow the seeds on top of the compost and cover with a thin layer of grit, mainly to stop them blowing away.