Is it in a pot or in the ground?

Its in the ground. Been in around 8+ years.

Sometimes this is simply down to insufficient water, especially if the weather is also very hot, but to some extent, many clematis varieties do brown off near the base during the summer -yours is just worse than most. I can see what looks like a couple of live leaves and a flower bud at the top left, which might indicate the problem was caused by drought like conditions a few weeks ago- if its planted within a few inches of a fence, and isn't 12-18 inches away, it'll also be in the rain shadow zone, meaning the roots don't receive as much water as other plants further away from the fence. Try watering more during dry spells - if that doesn't do it, cut it down as soon as it does this and let it regrow during the same season, if its early enough. You may get reduced flowering if you do this though, but it looks awful as it is.

Thank you Bamboo. It does have green on the top couple of feet and it is planted in a 6 inch border between patio and fence so your diagnosis is probably correct.... dehydration. I will make sure it's watered regularly next year and hope it recovers. Should I cut it down now or leave it until spring? You're right, it does look awful now.

I'd cut it down now because I live in London, but if you live in a colder region of the UK, best wait - cutting it may induce new growth which won't have time to harden off before frosts arrive. If you leave it, cut it down to about 8-10 inches from the ground in midwinter - there's some confusion about whether this is a prune group 2 or 3 clematis - some sites say group 2, others say 3 or 2-3, but group 3 ones are cut down in the way I describe.

Thanks again. I'll leave it until mid winter. I'm going away next week for 5 weeks so I won't have to look at it :)

Good plan!