Clematis can't attach to the fence without something narrower for them to climb on. Wire, netting, fishing line, chicken wire etc.

They can be trained horizontally but it is going to require work on your part to guide them.

Sweet Autumn is a huge grower and will overwhelm anything in it's path.

I have Arctic Queen growing on a wire fence and it does well but as Buyorsell said, it needed lots training. When left alone too long it will go air born and flop over the other side on you.

Autumn Joy definitely needs a big structure to climb, not only does it get amazingly tall, it becomes very heavy as well over time. I've seen pics where it climbed a trellis that was 10 foot tall, into a tree behind it and out over the roof of a garage after that. Beautiful but prolific to the extreme.

Thank you Buyorsell and tracyvine for your replies. This morning when I saw a tendril bent around one of the fence posts I just knew it was not going to work but I would not have thought of the chicken wire, so many thanks. Now then, while there is still not much growth I shall unwind the vines from the fence and tack up some chicken wire. It should not be too noticeable once the vines get going.

That Autumn Joy must be amazing to see, but how on earth do you prune/trim out the dead wood!!

Sweet Autumn Clematis is normally pruned almost to the ground every year.