Posted: 04/08/2016 at 07:54
If you have problems with mildew Newb, it's usually down to lack of moisture and poor air flow, so make sure your clematis are really well watered, especially if the soil's light, and allow them plenty of room to grow. A good mulch will help prevent moisture loss in dry spells too. Add some in late winter/early spring as well, when the new shots emerge, and that will also benefit them
Cutting them back will do no harm - it will encourage more new stems from below ground. The only sacrifice is some flowers from Group 2s which you may have got in the near future. All will be well next year.