Wednesday 19th June 11.30am
This is more of a useful change than a destination as it's served by the complimentary colours of the Jubilee, Victoria and Piccadilly lines. As for a destination, it really only serves Green Park, although it is next to the Ritz but I don't suppose people going to stay there arrive by Tube do they? Inside it's a bit of a maze, which is what you'd expect when three different lines were built at three different times, but outside it has a rather lovely entrance from the Park.
Wikipedia entry here.