...I noticed it today on my way to the doc's surgery (ill with flu, so shrouded in winter coat and blinking into the sunshine like a mole).
Birkenstocks, leggings and a tunic top. Sort of floaty, brightly coloured, suspiciously Monsoon-looking tunic tops. Maybe a few Per Una monstrosities, even. And yes, bums were generally well covered. Long black leggings. Almost all had white or beige Birkies on.
Practically every 30-40 something woman I passed in the street had this uniform on. At least half a dozen.
I swear it didn't exist last year (?). I think leggings have replaced linen trousers / culottes as summery mum wear.