I am removing a clematis called Huku Ookan because it is producing distorted flowers and it seems that there is something fudamentally worng with it as the distortion has been getting worse every year

Is it possible to have a closer look at the Huku Ookan Clematis?

If that's Dr. Ruppel on the left of that arch, I'm a monkey's uncle, Bernard! Dr. Ruppell should be deepish pink with a carmine bar on each petal. Whatever that is, its very nice anyway though.
You need something from the Jackmanii or Viticella group - these flower mid to late summer through till autumn, and will need cutting down to 2 buds from the ground every year by mid February (snow or no snow, ice or not, go out and do it anyway). have a look at Comtesse du Bouchaud, Ernest Markham, Etoile Violette, Gipsy Queen, Hagley Hybrid (prefers light shade), Rouge Cardinal, Victoria, Ville de Lyone, Margaret Hunt, Perle D'Azur, Lady Betty Balfour, John Paul II, Victoria or even Jackmanii.

Nickyt08, I have now added a photo of the distorted bud in answer to your request
Bamboo, you are most certainly not a monkey's uncle or any other animal relation. I have got my labels mixed up and Dr Ruppel is away blooming beautifully in another part of the garden. The correct name for the white one is Mrs George Jackman!
Thanks for your suggestions - I've already got Comtesse du Bouchard, but will investigate the rest with great interest.