Pruning wasn't carried out after this plant's early spring flowering. Is it possible to prune before the Autumn, say September as although it's quite healthy it's getting quite leggy. It's in a large pot facing East up against a fence and trellis. In the past I have covered it with fleece during the coldest months even though it's evergreen but I do get a lot of dead leaves and branches throughout the year. I water regularly and wondered if this is normal shedding for this plant.

Cartmanii is a group one clematis, only light pruning to shape. It forms flowers on the previous year growth. If you cut it in September you risking to have no flowers next spring. It is better to cut it right after flowering.