Astrantia, also known as masterwort and Hattie's pincushion, which I think is a wonderful name for this starry perennial flower.
There is so much to see in Tom Stuart-Smith's own garden, but it's only open for three hours once a year through the NGS so it's a race against time - and the huge numbers of visitors (surely many more than last year, and certainly vastly more than when I first visited in 2005).
Since last June, TS-S has created a prairie field (similar to, and using the same expertise as, the ones at Wisley and around the Olympic Park) and it's spectacularly good. I find these prairies quite beautiful: natural, restful, informal, with a gentle, swishing movement, and masses of interest and dots of colour once your eye settles, calms, and starts looking properly. The star of the prairie yesterday was the Carthusian pink (Dianthus carthusianorum) with its tiny magenta flowers that rise high above its spiky leaves and hover in the air (below).
And the astrantias next to the corten steel tanks were just right. Like the whole garden, really.