We decided to watch for a while and try to get a better look, what we found was a small nesting colony of sand martins. If you look closely in the photograph below you can see a row of nesting holes in the top of the cliff.
We watched the parent birds bringing in food for a while then as we were leaving down a small country lane we spotted the adult birds hunting for food, skimming the hedge rows. It was amazing to watch and at times they just passed above our heads. I took a short video clip to share with you here.