We've also looked after the Rosetta Stone pretty well
And how relevant is that? Your treatment of the marbles is downright criminal.
This whole 'give it back' idea is political correctness.
English didn't exist when the Rosetta stone was carved. How is "English" heritage perserved by having this artifact? Likewise, when the marbles were being carved, the typical Brit was dressed in the Bear skins and painting his face with blue woad. Just what does that British ancestor has in common with ancestors of the ancient Greeks?
When will the Brits stop hijacking someone else's culture and parade it around as if it was theirs? You don't expect the Greeks to lay claim to the Mary Rose, then why for heaven's sake do you insist in hijacking their culture?