Sometimes called Sweet Rocket, sometimes called Dame's Violet: Hesperis matronalis is a biennial/short lived perennial that has both white and pink flowered forms which sometimes cross pollinate to create a variant in subtle shades of lilac.
 The white form is dominant in the garden this year. Some years ago I planted H. matronalis at the back of a border and like plants often do it has wandered towards the light- by seeding itself into a more favourable position.

 Numerous visitors at the Open Gardens last weekend asked me to identify this plant in particular. I remarked several times that it doesn't seem to attract bees but is a noted butterfly plant (as seen in the pic above which I took last year).

 And then today I saw it being repeatedly visited by a large bumblebee or perhaps several of the same species (Bombus terrestris I think).
 Sweet Rocket aka Dame's Violet is luminous in full sun and seems to glow at twilight. It's one of those blooms that becomes especially fragrant as the sun goes down.