Dot
I have just looked a bit further into it and I think Ethel may be right. Trouble is, none of what I have found is terribly conclusive (and I never touch planning law, so I don't have any books here). The Town and Country (Permitted Development) Order 2005, seems to state that PP is needed in the circumstances I have said. However, elsewhere it seems to indicate that PP is not needed. In any event, if the flagpole is there legally, no further PP is needed to fly the national flag.
I would contact the planning department. Otherwise, Buildersmate may be on later and he seems to know loads about this stuff!