On a personal note, I have always thought a by-election should be automatically triggered when an MP or councillor defects to another party in between elections. Voters have elected a candidate under a party manifesto, and are clearly not getting what they thought when they placed their ‘X’ on the ballot paper. It’s not a common event, but it did happen in Hull last week.
Cllr Maureen Bristow, a Liberal Democrat councillor in Hull, defected to the Labour Party. This does not change the overall balance of the council, as the Liberal Democrats still hold an overall majority. Cllr Bristow is up for re-election next year.