Old posters at Oxford Circus tube

I've been meaning to do this for ages now. They're refurbishing Oxford Circus tube station, and they've revealed a whole load of old advertising posters underneath years worth of paper. Why these ones in particular have stood the test of time and not been peeled off or destroyed by everything that's been stuck over the top of them, I've no idea. But they make me smile whenever I go past them, especially the old H&M ones. The full set is on my Flickr page