Green Wing and Free Agents star Stephen Mangan is to play the lead role in BBC4's adaptation of the late Douglas Adams Dirk Gently's Holistic Detective Agency. 　　Mangan will be joined by Darren Boyd as his sidekick Richard MacDuff and Cold Feet and Friends actor Helen Baxendale, as MacDuff's girlfriend Susan. 　　Adapted by the Bafta-winning writer Howard Overman – who is also behind..