If you are certain that this is a virus then the only option is to remove and destroy the plants.

i aagree with owdboggy remove them both then dont plant anything there for a while then try again shame really we got 6 clematis and never seen that.i always thought they were very hardy against most things.but good luck and wish you all the best:)

We have about 100 Clematis and I have never seen any Virus on them either. All plants have virus in them , generally without producing any visible symptoms. Virus can be vectored by sap sucking insects, but the majority of Virus will not survive for long enough in a vector to be transferred from plant to plant.
The most common virus to affect Clematis produces green and distorted flowers, but the plants often recover the following season.
Are you a member of, or know someone who is, the RHS? You could then send a sample to them for expert analysis.

2 other possibilities spring to mind - glasshouse red spider mite can cause this appearance, and can affect garden plants, particularly in spring and early summer. Check the plant all over for signs of webbing or the little beasties themselves. The other possibility is magnesium deficiency, because in the pics above, it looks as if the yellow is confined to areas in between veins in the leaf - but usually, in magnesium deficiency, some yellowed areas will turn brown after a while.

I have not seen this before on clematis. Do you use any feed on your clematis because excessive application of a potassium-rich fertilizer can results in a magnesium deficiency, which in turn causes the plants to turn yellow between the veins. This often shows early in the season.

Was going to suggest watering it with some Epsom salts.

Thank you everyone, you have all given me lots of ideas as to whats wrong - I'm still not sure what to do & may send a sample to the RHS as Owdboggy has suggested & may be give it a dose of something!! A gardening friend has shown me that one of her plants has the same symptoms - may be it is the drought conditions we're experiencing in the Midlands & I know Clematis sometimes need more moisture than they naturally get, but I will just have to wait & see what happens. I have also put my question on the Clematis Society forum so I shall be interested to see what suggestions come from that & will keep you posted.

It will be interesting to know the answer, if the leaves were all green then the plant would look well.

Yes, do please let us know what The Clematis Society has to say. I was going to suggest asking them. I keep thinking I must join, but they want money.

I have have had the problem confirmed-it is a magnesium deficiency! So thank you to all esp those who did think it this. I didn't get this confirmation from the Clem Society - the only response on the forum was a possible iron deficiency though they could only rely on my description-owdboggy you don't need to join the society to use the forum so no money involved! I took a leaf into one of my local garden centres & the head plant lady looked it up in her book - I also looked in one of my books which also confirmed it - cos I'd been looking for a virus I missed the answer when I looked previously - motto is don't jump to conclusions!! I'm so happy I don't have to dig them up!

I did tell you it was a possibility in my earlier answer, Pixielady, but you never did tell me if you'd been feeding your plants and if so, what with? I'm wondering why they have a magnesium deficiency, if that's what it is.

Bamboo, sorry I didn't tell you that I don't feed many of my plants, except an occasional layer of my own garden compost, although I do know that clematis like to be fed so no, I don't know why they should suffer in this way either, but I have put some relevant feed in the soil now, don't know how long before any positive results. I did take note of your thoughts that it may have been this magnesium problem, but cos I was having a variety of ideas from different sources I wanted to consider all possibilities before I took any action. Just remembered that the nursery plant lady did respond to my 'why magnesium' with 'it could be leached out of the soil' depending on what sort we have - we do have a lot of sandstone chunks turning up on occasions.

Hmm, interesting - I'm not 100% convinced its magnesium shortage, but giving it a feed with epsom salts or similar should sort it out if it is that. Be nice if you could let us know if that was the problem.