Security at Westminster is regularly tightened. Armed police greet visitors and pass holders now have to use PINs to access doors inside Parliament.
So expect plenty of red faces among the authorities when they look up on the Commons roof and notice the arrival of several banner-displaying protesters against a third runway at Heathrow Airport.
As I write, the protesters from "Plane Stupid" are waving to the TV cameras from the roof of Parliament. At least one of them is wearing a stick-on Commons visitor pass. Oh, dear.