The only time to prune these is immediately after flowering, but a north facing fence is not an ideal situation for this clematis - it also doesn't do at all well in pots. It wants to get 35 feet high by about 10 feet wide, and it cannot produce sufficient root to make good growth contained in a pot, unfortunately, and that may be why its stunted. If you could transfer it to the ground, it has a much better chance of surviving healthily.

Thankyou for your reply.
Unfortunately I have no open ground to put it in. Maybe if I move it to a west wall it may stand a better chance.
The trellis I'm trying to cover is 9ft wide by 5ft high, north facing. I have tried a couple of different clematis, jasmine and honeysuckle but nothing seems to like it there. Any suggestions would be great please as I want to hide the greenhouse and compost bins. We did attached some willow to the back of the trellis but the wind blew it down.

When you say 'willow' do you mean the willow screening that you roll out and attach to a support? Did the wind blow down the trellis as well, or just the willow screen?

There are a few roses that will grow on a north wall. If you Google Roses north facing wall you'll find several suggestions. Before deciding do check the vigour and eventual height.