Is the island of Ireland where Brexit will matter most? Edward Stourton visits Londonderry, right on the Irish border, to explore what's at stake as the UK leaves the EU. Some locals fear the border across Ireland - as the EU's new external border - will harden, causing great practical and economic difficulty and even threatening the Northern Ireland peace process. Others say change the will matter far less, and that peace is now guaranteed. While people in Derry ask anxious questions, we'll hear too how policy makers in London and Dublin face a particular challenge in making Brexit work.