This magazine has an enlightening interview with the director of the National Gallery Gabriele Finaldi, in which he talks of the necessity of some religious knowledge in order to appreciate many of the works in the gallery.  One work in the gallery that Dr Finaldi singles out for a special look is the Wilton Diptych, about which he has this to say:

It’s a beautiful, mysterious picture, one that has miraculously survived the passage of time. It has survived iconoclasm and puritanism. It has come down to our time in a battered state, but the interior opens up a vision of a heavenly realm, which is unequalled. It is a very English take on what heaven is like: the King of England kneeling before the Virgin Mary and the heavenly court. Rather amazingly, and you have to look very, very close up to see this, the standard which is held above the Christ Child has a disc at the top. It’s actually an island with some small boats around it and it’s almost certain that it is England on a silver sea.