silene fimbriata

This plant should be cut back immediately after flowering; I'd trim it over a bit, but not much, and remember to cut it at the right time next year. If you cut it back by half next year, you may even get a second lot of flowers later in the summer.

thank you very much. when you say trim it over a bit, roughly how much should be left of the stem?

I don't know how big yours is, but I'd trim it lightly (though this may not make it look much better!) and I mean no more than a quarter of the length of the stems.