I dunno Daintiness - I'd probably be brutal if it were me, and have it look butchered now but get to where I want to go quicker in the end. Tattianna, these clematis flower now, and then spend the rest of the year growing the new wood on which next spring's flowers will form. So, the ideal time to prune is as soon as this year's flowering is finished (or sooner if you're not bothered about chopping them off since you can't see them anyway). Generally, people cut back in stages as described above in order to avoid the plant looking scalped for a few months and be a bit gentler on the plant, but if you don't mind effectively starting from scratch you can cut back the lot to within a couple of feet of ground level. A mature montana won't flinch. Put in some wires or something on the fence, so you can spread out and tie on the new growth, giving you maximum chance of seeing next spring's blooms. It'll recover really quickly if it's well established - I've known them put on 8/9 feet of growth in a single season. Give its feet a good thick mulch of well-rotted manure and don't let it dry out this summer - this'll keep its feet cool, which clematis love, and the goodness will also seep down and feed it, which it'll need if its got to put on all that growth anew. And Bob's your uncle. If you tie it in nicely you'll probably find it gets to the size you want this year, or next at the latest, and then you can just do the staged pruning to keep it to size and keep it flowering from lower down from then on. That also increases your chances of getting a second flush of flowers in autumn, as you won't have chopped off all those bits every year.
