I adore blossom, but it is short-lived. Can anyone suggest a frothy white/pink perennial that is about knee-high? I can only think of Campanula lactiflora, but that doesn't flower till late July.

Shame its dry - Astilbe sprang to mind from the "frothy white/pink" bit.
Have a look at Tiarella Skids Variegated - pink tints on the leaves when they're new, flowers not much to look at, but are short spikes in white with a pink tip. Good groundcover in shade. Might take a pic, have one in a pot on my balcony with some flowers on it currently...

In my dappled shady bed I've got:
Aquilegias (Granny's Bonnets)
Lunaria (Honesty)
Fuchsias
Roses
as well as lots of foliage plants.

Many thanks Bamboo and Sid . . . I love the Tiarella Skids idea. There are already Aquilegias and Honesty in the dappled shade bed, and Sweet Rocket in April/May.

I just put the photo on of the one I've got in a pot, Buzzbub, if you've not seen it already somewhere else.

Try good old London pride, Saxifraga x urbium. It will grow in shade ( I have it under a north facing wall) and has pale pink frothy knee high flowers for most of the summer.There is also a variegated one for extra colour. Very easy to grow and spreads slowly, not invasive, also evergreen.