Last week’s by-election result in Chesham and Amersham was a slap in the face for Boris Johnson. Fortunately it was a figurative one, unlike the punishment dished out to Emmanuel Macron by a disgruntled voter the previous week during a presidential walkabout. But it’s the fate of Macron’s predecessor in the Elysee that should focus Conservative minds in the wake of their chastisement in Chesham.