Number 10’s reaction was as bone-headed as it was swift: “This result,” a Conservative Party spokesman said in response to Sarah Olney’s stunning victory in Richmond Park last night, “doesn’t change anything.”

Except it does, in lots of ways.

Of course, there is always a tendency for by-election victors to overstate the significance of their win. By-elections don’t, on their own, change governments. Each constituency is very different — what works in one by-election does not work in the next. I safely predict that the by-election in Sleaford and North Hykeham next week will be fought on different issues and with different outcomes.

The Brexit press will be quick to dismiss the voters of Richmond Park as a bunch of affluent luvvies who don’t have a clue how the rest of the country feels. How dare well-heeled, well-educated and well-off Londoners tell the rest of the country that hard Brexit is a bad idea? Who do these whingey liberal elitists from leafy south-west London think they are? What a bunch of Remoaners! Theresa May is quite right to ignore them, the Brexit press will screech; the result “doesn’t change anything” because hard Brexit is what we want and what we will get. And so on.

But Theresa May would be well advised to pause and think a little about what happened last night. Her cynical calculation not to stand a Conservative candidate against Zac Goldsmith, tacitly encouraging Conservative activists and MPs to support him, backfired. Many Conservative voters I met in Richmond did not like the tricksy way in which Goldsmith presented himself as an independent candidate in nothing more than name, openly admitting he was likely to rejoin the Conservatives if he won.

Richmond Park by-election 2016 - In pictures 16 show all Richmond Park by-election 2016 - In pictures 1/16 Liberal Democrat candidate Sarah Olney reacts as she is declared the winner of the Richmond Park by-election Getty Images 2/16 Zac Goldsmith looks dejected as he listens to newly-elected Liberal Democrat MP Sarah Olney speaking PA 3/16 Liberal Democrat candidate Sarah Olney with her husband Ben and party supporters celebrate after winning the Richmond Park by-election Yui Mok/PA 4/16 Zac Goldsmith speaking after being defeated Yui Mok/PA 5/16 Ballot papers are tipped on to a bench in front of count staff during the Richmond Park by-election count at Richmond Upon Thames College Lucy Young 6/16 A counter sorts ballot papers at the Richmond Park by-election count at Richmond Upon Thames College Chris J Ratcliffe/Getty Images 7/16 Liberal Democrat candidate Sarah Olney delivers her winning speech PA 8/16 Liberal Democrat candidate Sarah Olney and her husband Ben celebrate after winning the Richmond Park by-election Yui Mok/PA 9/16 Zac Goldsmith arrives at the Richmond Park by-election for the result Yui Mok/PA 10/16 Liberal Democrat candidate for Richmond Park Sarah Olney arrives in the count hall in Richmond Justin Tallis/AFP/Getty Images 11/16 Zac Goldsmith (left) looks down as the result is announced Yui Mok/PA 12/16 Zac Goldsmith speaks after being defeated in a by-election he triggered himself Yui Mok/PA 13/16 Zac Goldsmith's mother Lady Annabel Goldsmith (3rd left) consoles a dejected supporter, as his wife Alice Rothschild (left) listens to him speak Yui Mok/PA 14/16 Newly elected Liberal Democrat MP for Richmond Park Sarah Olney listens to Independent candidate Zac Goldsmith Justin Tallis/AFP/Getty Images 15/16 Zac Goldsmith shakes hands with the winner of the Richmond Park by-election, newly-elected Liberal Democrat MP Sarah Olney Yui Mok/PA 16/16 Zac Goldsmith leaving Richmond upon Thames College after defeat Yui Mok/PA 1/16 Liberal Democrat candidate Sarah Olney reacts as she is declared the winner of the Richmond Park by-election Getty Images 2/16 Zac Goldsmith looks dejected as he listens to newly-elected Liberal Democrat MP Sarah Olney speaking PA 3/16 Liberal Democrat candidate Sarah Olney with her husband Ben and party supporters celebrate after winning the Richmond Park by-election Yui Mok/PA 4/16 Zac Goldsmith speaking after being defeated Yui Mok/PA 5/16 Ballot papers are tipped on to a bench in front of count staff during the Richmond Park by-election count at Richmond Upon Thames College Lucy Young 6/16 A counter sorts ballot papers at the Richmond Park by-election count at Richmond Upon Thames College Chris J Ratcliffe/Getty Images 7/16 Liberal Democrat candidate Sarah Olney delivers her winning speech PA 8/16 Liberal Democrat candidate Sarah Olney and her husband Ben celebrate after winning the Richmond Park by-election Yui Mok/PA 9/16 Zac Goldsmith arrives at the Richmond Park by-election for the result Yui Mok/PA 10/16 Liberal Democrat candidate for Richmond Park Sarah Olney arrives in the count hall in Richmond Justin Tallis/AFP/Getty Images 11/16 Zac Goldsmith (left) looks down as the result is announced Yui Mok/PA 12/16 Zac Goldsmith speaks after being defeated in a by-election he triggered himself Yui Mok/PA 13/16 Zac Goldsmith's mother Lady Annabel Goldsmith (3rd left) consoles a dejected supporter, as his wife Alice Rothschild (left) listens to him speak Yui Mok/PA 14/16 Newly elected Liberal Democrat MP for Richmond Park Sarah Olney listens to Independent candidate Zac Goldsmith Justin Tallis/AFP/Getty Images 15/16 Zac Goldsmith shakes hands with the winner of the Richmond Park by-election, newly-elected Liberal Democrat MP Sarah Olney Yui Mok/PA 16/16 Zac Goldsmith leaving Richmond upon Thames College after defeat Yui Mok/PA

If she thought about it, she would have mixed reactions to the way Labour lost its deposit too. That outcome was a resounding rejection of the decision by Tom Watson and the other machine politicians in the Labour Party to stand a candidate in the hope that Goldsmith would keep the Lib-Dems out. Labour High Command knew that its candidate stood no chance in the election but was desperate to avoid the beginnings of a Lib-Dem parliamentary fightback. Theresa May will no doubt enjoy Labour’s lamentable performance but she will also be aware that if Labour voters were prepared to support Sarah Olney in Richmond, then they might also be prepared to do so in a swathe of seats in the south-west of England, so reassembling Lib-Dem majorities that were lost to the Conservatives last year. Labour’s collapse in Lib-Dem/Conservative marginal constituencies like Richmond is bad news for the Government.

But the biggest lesson for Theresa May inevitably relates to her plans for Brexit. At a packed public event last week in Richmond, at which Sarah Olney and I were invited to talk about Brexit, I was very struck by the mood. The room was full of moderate, mild- mannered, thoughtful folk, the overwhelming majority of whom had voted Remain on June 23. So — before the screechers in the Brexit press point this out — it was not exactly a representative audience of wider national opinion.

But the discussion was nonetheless revealing. There were a lot of questions about the arcane twists and turns of the case now before the Supreme Court concerning Article 50. Interestingly, no one flinched when I stated baldly that Article 50 wouldn’t be stopped under any circumstances, regardless of how Sarah or I will vote, because Labour has already given the Government its unconditional support to trigger it, and that it would be wrong for the unelected House of Lords to try to do so.

In other words, the crowd accepted — as do I — that the only way in which Brexit could, or should, be reversed in future is if the British public has a change of heart. A decision taken by the people can only be changed, in the end, by another decision by the people.

There was also a fair amount of discussion about how we could avoid throwing out the single-market baby with the EU bath water. Everyone at the event remembered that the Brexiteers refused to spell out with any clarity what Brexit would mean in practice. Johnson, Gove, Farage et al never published a manifesto laying out their collective, agreed view on whether Britain should be in or out of the single market, in or out of the customs union, and so on. So Sarah Olney, as she rightly stressed in her acceptance speech last night, has a strong mandate from the voters in Richmond to pursue the least economically self-harming version of Brexit.

But beyond Article 50 and the single market, what struck me most was the quiet but brewing anger at the way in which the Government and hardline Brexiteers are gratuitously denigrating people who voted to Remain and are unsettled by the shrill vitriol that is now spreading towards foreigners, the judges and anyone who dares question the wisdom of “hard” Brexit. It is as if the people of Richmond — not to mention more than 16 million other voters who supported Remain — are being told that their values, aspirations and needs are no longer legitimate. They are being summarily disenfranchised by the assertion that a narrow victory for Brexit in June means that their views no longer count.

So that is really the lesson of last night for Theresa May: she is the Prime Minister for the whole of the United Kingdom, not just for her party or the hardest of hard Brexiteers. She must reach out, urgently, to those millions of people who simply do not share the worldview of Farage, Gove and Fox. Yes, the Brexiteers won on June 23 — but with victory comes magnanimity and responsibility. It is now Theresa May’s duty to show a bit of both. Otherwise Richmond may turn out to be the first of many election upsets.