One of the best types belong to the Viticella group (Virgin's Bower Clematis). There are many selections of it and near hybrids which all flower from mid summer (some stretching into autumn too such as Polish Spirit).

They flower on new growth so the old stems can be hard pruned back in spring and they don't suffer from Clematis wilt either!

A few suggestions of it are below.

C. viticella 'Etoille Violette',
C. v. 'Abundance',
C. v. 'Madame Julia Correvon'
C. v. 'Alba Luxurians'
C. v. 'Polish Spirit'

Thanks very much. Sounds a definite possibility.