I'm suspicious because it's 48 hours after the attack and there were 8000 people in the building next to the alleged incident. The London tabloids would not have kept this quiet for so long.
Would they really tell them to remain in the building for at least 6 hours?
Imagine being the person being told to do this.
As poster redlady said the remain in the building part should be easy to confirm, 8,000 or so witnesses.