Suicide bombers do not buy return tickets. Theirs is a one-way trip. When four young men met at Luton railway station a week ago last Thursday, however, they gave every impression of going to London and coming back. They paid and displayed, leaving valid tickets on the windows of a Nissan Micra and a Fiat Bravo in the station car park. They boarded the 7.48am to London carrying return tickets.