Feb; is probably the best time to tidy up clems; if yours is established, a pruning of down to about 18" I think! it could be a Jackmanii although there are a few purple ones.
Some-one else more experienced than I will probably put you right ! but that is what I do! before the buds start to grow.

Hello, Mr president is a purple one that I trim back to a fat bud to a metre above base in February. Your purple late flowering clematis possibly belongs to this group. The early flowering clematis, tend to be pruned after flowering, to keep them tidy and contained as they will flower early on the shoots produced in the previous summer. Whatever you prune be careful not to go into the woody main branches - just the off-shoots and back to a bud.

Easy to remember rhyme. If it flowers before June then don't prune!

As everyone says, there's quite a few purple flowered ones. The way to tell which group yours belongs to is to work out quite when it flowers. If it doesn't start till June or July, and then flowers profusely for the next 3 months, then its a viticella group, and should be pruned down to 2 or 3 buds from the ground now.An example of this type of purple clematis would be Etoile violette, or Jackmannii.

On the other hand, if yours flowers late spring, with very large blooms, stops for a while and then starts again around July, with smaller blooms this time, then its not a viticella group, and doesn't need pruning. You CAN prune ifyou want to, now, but if you do, you will sacrifice the earlier, larger flowers, but still get the later, smaller ones. Examples of this type of clematis (and which are purple) are:- The President, Xerxes, Violet Charm, William Kennett, Lady Northcliffe (though the latter two are more lilac than purple)