This looks like Geranium sylvaticum 'Mayflower'
http://www.plantsforshade.co.u...
It might be a similar cultivar but the flowers look right from what I can see, this site can be trusted to have the correct photo..
http://plantsforshade.co.uk/ac...
Nikita is similar but not as good a match, so is Amy Doncaster.
There's a list of sylvaticum cultivars on this US site for sale.. many from Crug plants
http://geraniaceae.com/cgi-bin...
It could be the straight species Geranium sylvaticum ..
http://wildseed.co.uk/species/...
You can view a very wide range of Geranium on Crug Farm plants, these people are plant hunters who have often met up with Dan Hinkley of the former Heronwood.
http://mailorder.crug-farm.co....
From the leaf I can see at about 1 o'clock it looks like G. sylvaticum, I can find nothing else which comes close and I imagine there will be some variation with different forms.
