I have exactly the same on one of mine and posted a question on here last week. The advice to me was 'possibly lack of magnesium and to try Epsom Salts'. I applied 1/2 a cup around the plants and watered it in, you can but try.

I'm a bit suspicious about this Drc, as the leaves of my clematis and yours show no signs of yellowing between the leaf veins. However, I have no idea what else might be causing the problem but there seems to be a possible quick cure if it is magnesium deficiency as Bamboo suggests, published on the Internet by 'Which' that advocates spraying the foliage with a solution of Epsom salts 20g/litre. Apparently, adding it to the soil is only a long-term fix.
I'm going to try this and if it doesn't work, it might be worth throwing it back into the GoY pool, but since Bamboo is the only one who has responded to your query, he might have no other suggestions to offer.

Bernard the Epsom salts spray on the leaves has to be repeated several times. I am a great user of Epsom salts so I was more than willing to try it around the roots which is the way I have used it for over 30 years. I had this last year exactly the same on the same Clematis I removed all the affected leaves and it made no difference, but as this year I have loads of flowers.

Drc, this thing is a complete mystery to me and I am struggling a bit to understand it, for instance, why no yellowing of the leaves, why is only part of the plant affected, especially the older leaves, when the new ones are clear, and why, when there are other clematis very close by, are they completely unaffected. When you've used the Epsom salts spray before, has it made any difference?

I have exactly the same problem Bernard, mine is on the left hand side of the plant and on the older leaves.
Have you looked at my question photo posted 27th May? Although the leaves are not yellow I do think mine has a yellow tinge all over the plant? I have never given it Epsom salts before but its 12 years old and could be short of magnesium as suggested? Whatever it was did not kill it last year but I removed all the affected leaves and it looked awful so I have trying the Epsom salts first.
I have 9 other Clematis and they seem fine but only 2 are as old as this one - the others much younger.