I didn't know this one- what a beauty - on my wish list now!
Don't worry.
I looked it up for you and found this info:Its a group 3 which flowers late summer so you prune it in spring before it starts growing. Cut back all of the previous years stems to a pair of healthy buds at around 20cm (8") above ground level before it starts into growth in the spring.. Then apply a mulch of well rotted manure or garden compost to the base of the plant in order to protect the roots and conserve soil moisture.

Similar info on RHS site. It's a nice loooking plant.

This herbaceous clematis forms an open bush, that looks wonderful when allowed to drift through the mixed or herbaceous border. For several weeks from midsummer, it is covered with deliciously scented, hyacinth-like, lavender-blue flowers, which have a delicious scent and really stand out against the backdrop of the lobed green leaves.

Garden care: Cut back the stems to the base of the plant each winter and apply a mulch of well-rotted garden compost around the plant, avoiding the immediate crown. Use twiggy prunings, ring stakes or bamboo canes to provide support.