There have been few more powerful painters of landscape than John Constable. He brought a scale, ambition and impact to a subject long considered amongst the lowest forms of art. Constable is often celebrated as a nostalgic painter of a lost England but look a little harder, though, and you discover an intense and radical vision at work which changed the course of British art. As well as looking Constable’s most famous works, such as The Haywain and Flatford Mill, Tim Marlow explores lesser known works such as the expressive sketch for The Leaping Horse