
I think you will find that they were house martins. The ONLY reason I know is because we had the same thing happen at school the other week on the playing field (I am a lunchtime supervisor)
It was mine and Wendy's turn to walk around the school grounds. When we got to the playing field, we noticed masses of birds ...and I mean masses...there must have been upwards of 50.
They were swooping down very low and then going into a large oak tree time and time again. We walked closer and closer and the birds weren't in the slightest bit afraid. They swooped so close that we could have reached out and touched them.
I am assuming that there was a glut of insects in that area and that was what they were after. We stood there amongst all of these swooping, diving and twittering bitds for a good 10 minutes before walking off. Later on we went back and the birds were still there.
When I got home I looked up swallows , swifts and house martins and it appears that HM's are the ones that do this :)
Absolutely stunning sight!