David Bolam: Why was British hostage freed?

Islamic State is trying to goad western governments into a "boots on the ground" conflict to create a "good versus evil" war, a terrorism expert has said.

IS published a video on Friday apparently showing the murder of British aid volunteer Alan Henning - their fourth such video.

However, on Saturday it emerged that Libyan militants had released a British hostage, David Bolam.

Dr Evan Lawrence, a visiting lecturer in security and counter-terrorism at Staffordshire University, explains why IS differs from other militant groups.