I'm guessing that's a Clematis montana so it flowers in April/May with light pink blooms with yellow stamens?

What I would do with that big 'birds nest' is try to untangle some of it if that's possible. If not you could just leave it where it is to let it bloom which is 2 months away (if it's montana). If you prune it now you will lose the blooms.

When it's finished blooming (May time?), take a saw and cut the thick woody main stems back to about 6" above the ground. It will react by throwing vigorous shoots. These should be fanned out and trained against the wall as they come up and will form the new frame work. Even though they are young they will be very brittle so you need to be careful they don't break as you bend them. If it's a montana these shoots will grow several feet long so be prepared to use a ladder if you want it to cover much of the wall.

Thank you for your advice. the trouble is it's really in the way but I'll try to wait for the blooms first.