Large group migrating North?

Yesterday evening around sunset a large group of birds in the usual 'V' formation were flying North. There were about 20 or more small groups of about 10 to 15 in each flock. They did not look like Geese and were not making any noise, they had long slender wings like large gulls and appeared dark in colour. Any ideas what they may have been and why were they heading North at this time of year? BTW I'm living in Oxfordshire.