Why 42 days?

It seems only yesterday that Jacqui Smith was saying “I don’t know” when asked how long terror suspects should be detained without trial. Now it seems she has decided on 42 days. The government needed to reclaim the news agenda, and this was a button waiting to be pushed. Why not 56 days? Why not 35 days? The figure will have come not from the police or MI5 (who don’t arrest anyone) but from the Labour whips. And even this may be a big ask.