Zac Goldsmith was probably right to resign his seat and contest a by-election as an independent over the Government’s decision to give Heathrow expansion the green light.

Not necessarily because it is a good idea in itself, but because it was part of his promise to the voters of Richmond Park, and at the last election they upped his majority from 4,000 to 23,000.

Had he reneged on the deal he would be charged, not unfairly, with contributing towards the pervading mood of popular distrust of politicians, and four years with Heathrow making headlines, and with that broken promise hanging over him, may have turned the next election into a better opening for the Liberal Democrats even than this by-election.

It may not always be wise to keep unwise promises, but the promise and the decision to keep it nonetheless need to be judged on their separate merits.

The decision made, the Conservatives had only two sensible responses.

Option one, advocated by Tory MPs such as Alec Shelbrooke, was to cast the apostate into the outermost darkness and throw the kitchen sink at unseating him.

Satisfying as such a course of action might have been, it’s very difficult to see how it might possibly have yielded a good result for the Party.

If we set to one side the fantasy that a new Tory might have stormed the seat against the Liberal Democrats and Goldsmith on a pro-runway, pro-Brexit platform, or of running another anti-runway MP, we’re left either with two losing outcomes: failing to dent the incumbent whilst alienating him from the Party; or making a gift of the seat to Tim Farron.

Thus CCHQ has decided to go with option two: leave well alone and let Goldsmith try to hold the seat with as much informal support as individual Conservative activists wish to give him.

As Shelbrooke rightly notes, the formal loss of the member for Richmond Park from the Conservatives will cut the Government’s paper majority from 12 to 10. But as left-leaning commentators like Andrew Rawnsley are quick to note, there are opposition MPs and there are opposition MPs.

Barring a wholesale philosophy transplant Goldsmith will probably vote with the Government most of the time. Crucially, as a Brexiteer, Theresa May should be able to depend on him on EU business.

An independent but friendly Goldsmith was probably the best outcome CCHQ could realistically hope for from Richmond, and the Government’s majority is too thin to risk seeing an actively hostile MP elected for principle’s sake.

This way everyone’s bridges remain unburnt, and the possibility remains that the MP for Richmond Park may retake the Conservative whip at a later date.