Soapwort - Saponaria Officinalis
This is an easy to grow perennial, which reaches the height of around 3 ft. It produces clusters of rose-pink flowers in late July. It looks best with some support to keep it upright. The rhizomes contain saponin, which can be used to make soap - hence the name. It likes well composted soil in partial shade.
Sow the seed in late winter (January onwards) they will take about one month to germinate at room temperature.