Two police vans waited expectantly near the front entrance. Officers patrolled the pavements while suited security men with ear pieces stood stern-faced, casting suspicious looks at those approaching. The location in east London had not been disclosed until that morning but no chances were being taken.It was not for a visiting dignitary or even an embassy of a country in conflict that all this was deemed necessary but the Nova exhibition, a commemoration of the 378 people massacred at a music festival on 7 October along with the 44 taken as hostages and the 19 of those who died in...