hi my first thought was snow in summer, sorry cant recall its botanical name. then the aubretias usually purple flowers about now, Arabis pinks/whites. Alyssum saxitalis yellow flowers.

there are sub-shrubs such as parahebe that will creep over areas like this. heathers might work too.

Parahebe is lovely and it flowers from May to...well mine was still flowering at Christmas last year. And thats in an almost totally sunless situation. Vinca minor would be lovely there too...there are a couple of white flowered varieties which would work well with the Parahebe. :) Both evergreen.