But why 42?

As Gordon Brown prepares to face what could be the Waterloo of his premiership something has suddenly started to bug me.

In terms of the vote tomorrow on 42 days detention without charge many valid points involving erosion of our civil liberties have been made. But no one has raised the question of why 42? Blair went for 90. Beyond the fact that Brown doesn't want to push his luck, having seen his more popular predecessor defeated on 90, why just slightly less than half the previous attempt? Why not 45? Or some other nice round number? 50? 40?

Could our country's leader be a closet Douglas Adam fan?