A couple of additional things... all clematis like shaded roots, so be sure the plant has lots of mulch, such as shredded bark or peat moss to shade the roots and keep the soil cool. Also, take a sheet of plain white paper and, holding the sheet under a branch, shake the branch fairly briskly. If there are any pin head size dots that scamper across the sheet, you have spider mites and need to treat the plant with a good miticide... For sure, don't destroy it... best of luck!
I have the same problem, starting at the base of the plant and gradually working upwards. A nurseryman friend claims it's a clematis wilt virus, and the plant should be cut down, dug out, and burned. But it flowers beautifully before the wilt sets in, so I'm leaving it be. Apparently, hybrid varieties are more susceptible to this than some of the original stock plants, like good old Montana
dear all,
many thanks for your replies, they were very helpful.
the roots are covered by mulch.
i may have over watered, as i was watering them as soon as we had a dry day (which is not many i know) :-)
i think i will leave it and just hope that next year it recovers. i am a bit reluctant to purchase another at the moment.