Try phoning up Man Utd themselves, I have heard that some of the players land close to the ground every now and again (don't know where)........but then again, this was through a friend of a friend, so I don't know how accurate this info is.
On a negative note, I have asked the local councils on a number of occasions for permission to land in this area, on ground owned by them and it's always the same outcome, I am put through to the "Health and Safety Dept" and none of them will put thier name to paper.
Good luck
C.