“The National Trust is now opening a permanent commemoration of the charter, in the form of a new work by Mark Wallinger. Writ in Water, as it is called, is an entire building. The artist – who won the 2007 Turner prize and whose statue of Christ, Ecce Homo, was the first contemporary sculpture on Trafalgar Square’s fourth plinth – teamed up with architect James Lowe to produce what is his largest-scale single work to date.” Charlotte Higgins pays a visit.