Epilobium angustifolium (probably) ....
I'm no wildflower expert, but I'm fairly sure this is rosebay willow herb. Though when I looked in the book for the proper name I saw several other possibilities.
I stopped on the hill on my way home and took some shots of what was growing on the bank. But there was a stiff breeze blowing and everything was waving about. So I picked the last little floret on this one (I'm sure its siblings are already sprouting, there's no shortage) and retreated into the car to blip it.
The car makes quite a good studio - indirect light and plenty of armrests etc to steady the camera. Might try that again.
Thanks to Miranda1008 and osuzanna for hosting Wildflower Week and Tiny Tuesday.