I've been told it's cow parsley and so have edited the blog. I've always called this plant hogweed though, because it looks the same as giant hogweed...but not as big.
I find one way to help identify it is to mush a a few of the flowers in your fingertips, cow parsley has a very distinct aniseed smell to it.
It also has no 'beards' under the flowers (the flower head cases which hang like hairy beards on other similar plants)