T he nightmare that unravelled the lives of Sally Lane and John Letts started with a single phone call more than 12 years ago.

The British-Canadian parents had initially been relieved when their eldest son, Jack , showed a natural proficiency in Arabic.

Their introverted teenager, with an apologetic smile, had been struggling with severe Obsessive Compulsive Disorder at school in a quiet middle-class neighbourhood of Oxford. He converted to Islam and asked to spend part of his gap year studying Arabic in Kuwait.

Just a few weeks into the course, in August 2014, he called home.…