Look at the detail on the one caryatid that Lord Elgin carried back to England: Now, look at her sisters who have remained on the famous porch of the Erechtheion, especially the one on the corner (the lady in the back is a concrete replacement for the one Lord Elgin took). Their faces have been almost obliterated by the corrosive air.
Q.E.D.
This guy needs a refresher in capitalism. If Britain owns the marbles (and they do), Britain should have total control over where the marbles ought to be.
This case really opens up a whole can of worms, a little like reparations. The ruler of Greece, in 1801, gave the marbles to an Englishman. That's legal transfer of ownership. A great deal of art in the world's museums is there by donation. If it is decided that the heirs of the donor can say "Changed my mind -- give the art back, please" then the contents of the world's museums will quickly revert back into private hands. And then be sold to museums or (more likely) rich private collectors.