When the flower stems start to get scruffy looking, with blooms only at the end, cut them back to where new growth is coming out, just below where the earliest flowers were. At the same time, I would give them a complete fertilizer, such as growmore, to encourage new growth. The second set of flower stems will be shorter, and not as graceful, but it should be possible to keep it blooming until frost.

I've always found them to be tough plants and when i've cut them hard back they've produced new growth - they'll flower until autumn anyway so you'll still have time for flowers.
When i've discovered mine have been smothered by something i've given it space and it's shot away - so again, quite strong plants.
This has applied to all sized varieties.

Thanks Tugbrethil and Louise for your advice. Unfortunately Louise, in our garden ,apart from one small area it is all cottage garden where all flowers are mostly tall and very close together so smaller plants i now put in pots.All the borders are only in the sun for a few hours because of being shaded by the beech trees and scots pines.We both love our garden but oh,do I yearn for a sun drenched area.

Come and share mine then, Rose :-))
It's a very dry and sunny garden - too dry :-(

I would love to Louise thanks. I suppose all of us have some part of the garden we're not happy with.