He was interviewed in the belief that he had highly sensitive or stolen information. He stayed with a known film maker who was working on this issue, and is the boyfrind of the guardian reporter, he admitted carrying journalistic materials, and so there was more than reasonable suspicion that would allow him to have been arrested under the Official Secrets Act or some other legislation.
Fair play to the security servcies
except that if you do hold someone for any time you should be very sure what legislation you do it under. You cant just hold someone and then pick and choose your grounds after. It was anti-terror legislation they used.