Category 2 clematis pruning.

This is my neighbours clematis with it flowering now I assume category 2. As you see it's getting a bit sparse at the bottom, this is one I'm still yet to get my head around, so when do you cut back an by how much?

 

You don't have to prune group 2 but you can give them a light trim of stems going the wrong way . The bare stems / no flowers at the base is common not with just clematis but with nearly all climbing plants .

They can be cut back hard to the lowest buds in spring if it getting out of hand .

 

Thanks @Perki hopefully I can digest that.