On my Thames Towpath run today watched a Common Gull attacking a Grey Heron. Quite a crowd gathered as it went on for at least 10 mins. The gull was divebombing like a falcon, and then occasionally swooping in along the water 'under the radar'. If it attacked from the front, the Heron would attack back with its bill, but it didnt see the attacks coming from other directions and yelped with shock whilst ducking.
Eventually the gull gave up, and the heron decided it would then move on anyhow.