Great Masterwort (aka Pink Masterwort) (Astrantia major) is not native to Britain, and any occurrences are probably garden escapees. Some of the examples shown here were in a lone patch in the heavily grassed verges alongside the Worcester and Birmingham Canal on Midsummers day, 2006, an exceptionally warm year; indeed, the warmest for 350 years! [there were more records to beat that following 2006 - your Author, 2017].
There are two species of Astrantia, Astrantia minor, largely un-branched up to 70cm tall, and the largely similar but sturdier and branched Astrantia Major, which is up to 1m tall with an umbellifer-type canopy of flowers. The specimens shown above are branched therefore Astrantia Major. The conspicuous white 'petals' are bracteoles, the real flowers are within and much smaller, but still white (with perhaps a pink tinge).
Not to be confused with :
Masterwort (Imperatoria ostruthium) [a plant with similar name which is also an umbellifer, but not in the same genus]
If the reader thinks that some of the florets in the umbels look different to others, there are two types on the same plant: pedicellate (stalked) male florets and bisexual florets.
There are three or four sub-species of Astrantia major, all garden flowers, most of which can escape into the wild:
The above photos may represent several of these slightly differing sub-species.
Great Masterwort (Astrantia major ssp. major) with most bracteoles less than 15mm and with 3 green veins.
Great Masterwort (Astrantia major ssp. carinthiaca) more popular in gardens than the former, bracteoles larger at 15-22mm, still with 3 green veins.
Great Masterwort (Astrantia major ssp. elatior) which is grown in gardens and may escape in the future. Has 5-veined bracteoles and has papillose fruits (covered in small nipples).
Great Masterwort (Astrantia major ssp. maxima) which is most commonly grown in gardens and may escape in the future. Has potentially the largest bracteoles at 10-30mm long which have inconspicuous cross-veins.