Anyone who is selling seeds for a named cultivar - like 'Princess Diana' - is yanking your chain!! This is a hybrid and will not come true to type from seed. You may get some that will resemble the mother plant but many that won't and whatever is produced cannot be called 'Princess Diana'.

True species clematis are the only types that will come true to form from seed. So no culitvars!

GG 48 is totally correct; hybrids don't ever come true from seed, and though there is a chance it may resemble the parent, you may get something with flowers that are a different form or different color, and the vine may have a different habit. So if you want C. Princess Diana, you either need a plant that you can root a branch of or you need to start one from a cutting or you need to purchase a plant.