White Wagtails don't generally breed in the UK and certainly not in Greater Manchester. Juveniles of White and Pied Wagtails are extremely similar in plumage and in all likelhood your bird was a juvenile Pied Wagtail.
Ian's most likely correct about the bird being a juvenile Pied wagtail Mike as there were at least 5 of the youngsters flying around Rindle and Astley moss over the weekend.
the sighting occured today about 4.00pm on Astley Moss. it was on the track bend past the feeding station off rindle rd. it was definately an adult wagtail but was totaly grey with a black tail. i saw it and another chap who appeared quite an experienced bird watcher, but we both were at a loss. anybody out there with any ideas, regards mike
-- Edited by Ian McKerchar on Monday 31st of May 2010 05:20:33 PM