Fortunately they continued their activities for a while after we had returned home from our afternoon perambulations so I was able to stand in the garden with the 50D and its 100-400mm lens. It was quite a feat to keep any birds in the viewfinder, never mind about focussed. As the gulls were flying all over the place I left the lens on auto focus though I did shorten the focussing range so it didn't have to hunt too much.
It looks to me as though the flock was Black Headed Gulls, or at least all those I managed to get in reasonable focus were.
While we were out and about I took a short piece of video with the Lumix TZ7: