It is a tribute to the political skills and popularity of the late Dame Cheryl Gillan that the electoral dam did not burst and inundate the Conservatives in Chesham and Amersham at the last general election.

On Thursday, with her no longer there to contain public resentment, came the deluge. Most by-elections are transient phenomena with a little long-term effect on public policy; a few are serious warnings of disaster to come unless governments change course. Chesham and Amersham belong to the latter category.