Rather depends on how close to Heathrow you want to be but Twickfolk is geographically the closest and they have Suntrap playing that Sunday, on the other side of London Walthamstow Folk Club have Sam Carter (http://walthamstowfolk.co.uk/), and for something a little different Green Note in Camden have a storytelling night (http://www.greennote.co.uk/index.php?option=com_content&task=view&id=1856), the latter two are within easy distance of the nearest tube!