★★★★☆: Works on paper can often be seen as room-filler when a gallery can’t quite muster enough ‘proper works’, paintings and sculptures, to fill their space. They are normally objects that exist primarily in the realm of art historians, tools for them to unlock the secrets of an artist’s grander works. However, in the eternal struggle to find an original angle on that godfather of modernism, Pablo Picasso, the RA have dedicated an enormous show just for his works on and using paper.