grey wagtail?

backblipping for yesterday, in rather a hurry as going out in 5 mins and not even taken any photos today yet! We stopped of at Foxton Locks on the way home yesterday, where they have an impressive series of locks on the Grand Union canal. What caught my attention was a couple of little yellow birds who were darting about and catching flies over the water. They were really too far away and too fast for me!  I did manage to get some in flight, but rather cropped. They were flying up to a nest concealed in the wall and I was surprised once I lightened the photo to see tiny chicks in it.  I think they might be grey wagtails but not sure.