Neil MacGregor has in the past said that restitution claims against his museum have not been made officially, or in the correct way. It seems though that even if they are, it makes little difference to how they are dealt with.
Time Magazine Blogs 
November 13, 2007 8:43
Between a Rock and a Hard Place
Just two weeks ago Neil MacGregor, director of the British Museum, was telling me that Egypt’s loan request to his museum for the Rosetta Stone, which it wants temporarily for the opening of Cairo’s Grand Museum in 2012, was made in precisely the right way. (This in contrast to Greek attempts to borrow the Elgin Marbles, which get sticky because the Greeks won’t acknowledge first that the Trustees of the British Museum are the marbles’ rightful owners.) Let the record show that MacGregor wasn’t prepared yet to say that his museum would say yes to the Egyptians either. Now Zawi Hawass, Egypt’s very high profile culture minister, is complaining that the Brits are stalling.
I think this is called conducting negotiations in public.