I'd have said they were more likely long tailed tits if they were in your garden, unless you've got a stream running through it. Pied wagtails are common near water and, would you believe, in car parks and industrial estates. Lovely little buggers.
They're common outside No 7. There's a tree they roost in. Hundreds possibly thousands of them. Actually two trees. The second is sparsely populated with n0bhead pied wagtails that no one in the first tree likes
Smallish. Black and white. Identified then by the fact that they wag their tails. If they fit all these criteria, they will be pied wagtails, for sure. Pretty common.
I have PiedWagtails and Yellow Wagtails in my garden throughout the year but I live out in the countryside.