I think it is vipers bugloss, Echium vulgare.
It is widespread in England and Wales, but local in Scotland and Ireland. It flowers from June-August.
There are variants on the typical species, and yours seems a bit pale - so I can't be sure, especially considering the possible bird-seed source.
Thanks for your reply, but I think from further research (on the part of my wife, a Librarian) that it is actually Phacelia tanacetifolia, the foliage is correct as well as the flowers. It is used as green manure in some parts of the world. It probably came from the birdseed.
The flowers are quite like Viper's Bugloss but the leaves are very different. E. vulgare leaves (or the only ones i've seen) are plain and lanceolate, while the plant in the original has very divided leaves.
Unless of course it is one of the varieties that Mike mentions???
Here's a Viper's bugloss pic from the internet, showing the leaves, and a poorer one with my old camera showing the whole plant.
Don't have a clue what else it could be though, sorry!