At my age I tend not to dwell on anniversaries. But I have just celebrated an anniversary that gives me nothing but pleasure: 35 years since my victory in the Crosby by-election.

Just occasionally, by-elections make the political weather. With a Conservative government lurching to the hard right and Labour being led by an unelectable extremist (who says history never repeats itself?), suddenly the entire political landscape opened up to the Liberal/ SDP Alliance.

Alas, the Falklands war halted the resurgence in centre-ground, economically credible, socially-caring, pro-European politics. But by-elections have the power to stop divisive Prime Ministers in their tracks. Our victory at Ribble Valley, for instance, killed the poll tax. And on Thursday a victory for Liberal Democrat Sarah Olney in Richmond Park and North Kingston has the potential to kill an even greater evil: Hard Brexit.

The battle against arch-Brexiteer Zac Goldsmith is seen by everyone bar Goldsmith as the Brexit by-election. A recent poll showed that 90 per cent of the British public want to remain in the Single Market, yet Theresa May is hell-bent on the economically suicidal course of hauling Britain out of the world’s most lucrative market. Every tariff against British goods, every regulation that we will have no power to influence will be a hammer blow to British jobs. Theresa May seems to be sleep walking towards a trade war – and it will be ordinary citizens, not the global elite, who will pay the heaviest price.

Over 5,000 people in Richmond Park and North Kingston work in the City, and not all as merchant bankers, but in support services and related professions. What happens to the City if Britain loses its passporting rights?

Another 8,000 local people work in the public sector, who will see their modest pay packets nibbled away at by Brexit inflation.

Think of the families, think of the livelihoods, depending on us being able to trade freely.

If it were a Labour government purposefully embarking on a course that will leave a 220bn Brexit black hole in the public finances, the outcry would be deafening; but without a functioning Official Opposition, the Tories have got away with it. Until now.

Liberal Democrats have spoken to more than 40,000 people in Richmond Park and their demand that the public be allowed to vote on both the destination as well as the departure from Europe has struck a chord. Some 72 per cent of voters were for Remain in Richmond. They don’t want the Ukip-backed Goldsmith returned to parliament, where he has made clear he will push for Hard Brexit.

In this internationalist constituency, voters also recoil from the Conservative tactic of using European citizens who have contributed so much to Britain’s economy and society as little more than pawns in a hostage negotiation.

Not only is this morally wrong, it is politically tin-eared. Theresa May treats all EU negotiations as being transactional. They aren’t: Europe shares the ideal of countries working together in peace and co-operation. Angela Merkel has already made clear that freedom of movement is more important to her than car sales.

So far Theresa May seems to think all the political pressure is from the right - pressure from the isolationists in her own party who for three decades have disparaged Europe at every opportunity and now wonder why the EU isn’t prepared to let Britain have its cake and eat it.

But 48 per cent of Britons voted to remain. Many others only voted Leave because they were told we could stay in the Single Market, that other markets would instantly open up to us, and because a fanciful Brexit bonus would give us an extra £350m a week for the NHS.

None of it has happened. The government won’t reveal its plan – except by accident on notepads outside Number 10 – because it doesn’t have one.