Well I always deadhead these the way you describe, just behind the flower, but I do it as the flowers fade, otherwise new leaves, which appear beyond the flower, carry on growing and you end up removing grown on foliage. You don't have to deadhead, but I don't like the bare bit of stem left behind after the flower fades away. It is really a bit late to do it now if there's plenty of leaf growth beyond where the flower was - its equivalent to a proper prune, winter is coming soon, and these plants aren't entirely hardy, so you don't want to force new growth at this time of year. When did it finish flowering?

I suggest you leave well alone - if you want to prune back next year, do it as soon as the flowers finish, and reshape the whole thing if you want to then. If we knew we weren't getting a proper winter (like the last two) then it'd be fine, but we don't know what we'll get... and you may be cutting off growth that would flower next year if you do it now.