Are the stems with blackened leaves still healthy? Presumably they are otherwise the topgrowth would die. These plants do tend to have bare stems at the base as they age, so leaves turning brown and dropping isn't that unusual, but black leaves is. Any chance of a photograph?

Could we be talking clematis wilt here, Could it be down to last winters harsh weather, i say this as i have noted certain plants that were affected by last winters cold perked up and did well only for all the new shoots to dieback, other plants that were not affected have struglled later in the season, i note that i have seen two clematis armandii that have had alot of browning leaves similar to yours and looking unsightly, but on one of them a couple of new shoots have appeared from ground level, i have though not seen armandiis affected by c, wilt.

Can't be Wilt, Julien, this is a species clematis and they don't suffer from it, its only the large flowered hybrids that get Wilt.

Many thanks for helpful comments. There are 2 green leaves about 2ft from ground - then bare followed but lots of leathery brown leaves. Perhaps black was an exageration! But as I said the other half of the plant looks green and glossy and is still growing. Perhaps I will leave it over winter, let it flower in spring then give it a good 'haircut' to see if that brings the growth back from the bottom of the plant.

That is precisely what I'd suggest. My book says this one doesn't require pruning, and a light tidy up is all that's needed, but in my experience, in smaller gardens, that just means all the leaves and flowers are up in the nearest tree with lots of bare stems at eye level...
And another piece of advice - forget the Epsom Salts - only necessary when magnesium shortage is a problem and the symptoms for that are not anything you describe.