This is Iain Martin’s newsletter for Reaction subscribers.

Well, this has been quite the week for the upending of assumptions. How the mighty are fallen, a bit.

“Boris,” a very senior Conservative who knows his politics told me early in the week, “is in such an unbelievably strong position.” Not a cloud on the horizon. Nothing touches him. Nothing sticks – until it does.

On Thursday, the voters of Chesham and Amersham in Metroland, in the semi-rurual, demi-suburbs outside London took a baseball bat to that notion and to the Conservative party’s majority there. A true blue seat is now a Lib Dem stronghold by a majority of 8,000 following a by-election.