Why does the head of Goldman Sachs in London want to have tea with John McDonnell? It seems unlikely that he hopes to fill a gap in his autograph collection, or to secure a selfie to use as his new Facebook profile pic. Presumably Richard Gnodde hopes to lobby the Shadow Chancellor in some way – to encourage him to a more positive understanding of the work the City does, perhaps, or to persuade him of the merits of more business-friendly policies.

Can Gnodde really believe that this is a realistic prospect? The man he wants to meet is a self-described Marxist, setting fiscal policy for the most left-wing Labour leadership in living memory, and has spent years as the key ally of an Opposition leader who only two weeks ago told the financial services sector that his Party is “a threat” to them and their work. Goldman’s man in London stands a better chance of getting that selfie than of securing any positive change in McDonnell’s view of the world – trying to cosy up to Labour now is an act of absurd desperation.

Some in business know this. Indeed, some in Goldman Sachs know this – one of Gnodde’s colleagues recently warned that Corbynism would transform Britain into “Cuba without the sunshine”. But even among those who think it, the number willing to say it out loud in public are still relatively few and far between.

Why might that be?

To an extent, I suspect it’s because there’s still some degree of fantasy at play about the true nature of the practicalities of socialism. Many people have forgotten – understandably, given how long ago it was – how huge and hard the task of freeing up the economy was in the first place. So there’s a limited practical understanding of what mass nationalisations, vast expansion in borrowing, large tax hikes and strict capital controls to shore up the currency would actually look and feel like.

That’s twinned with a naivety about the people involved. Corbyn and McDonnell do a pretty good job of performing their great uncle act, posing as Westminster’s answer to Statler and Waldorf – albeit with a little red book in their inside pocket. Their advisers, and their apologists in the media, have done an equally good job of stirring up doubts about what they really believe. Disregard everything Jeremy and John said for the previous three decades, we’re told, and look at the statements they issue now. See, they’re suitably vague as to make it theoretically possible that actually they’re merely Swedish social democrats. Yes, their careers and their teams are saturated in the politics of the far left, but it’s all going to be ok, nothing to worry about.

The final, key, element is the implicit threat of Corbyn’s accompanying angry mob. This morning, James Frayne wrote about the troubling and misguided tendency of businesses to roll over when confronted by loud but fringe online activists. If that’s the effect of a couple of hundred people assailing Pizza Hut about their newspaper adverts, it’s no surprise that plenty of businesses stay silent about the dangers of Corbynism rather than risk the wrath of the mobs of thousands directed by Momentum, the Canary et al.

The difference, of course, is that while a pizza chain might be right in thinking that if it surrenders on advertising in the Mail or the Sun then the problem will go away, the opposite is true when it comes to indulging Corbyn’s Labour Party. Keep quiet about their threat to jobs, growth, tax revenues and public services, and they might not attack you online today, but you increase the chance that they will get to attack you, and all of us, with the power of the state if they get elected.

Business should do more – as individuals, as companies, or as a group – to make quite clear what the Opposition would do to this country if they get the chance.

That doesn’t have to mean involvement with or endorsement of the Conservative Party. Though, as I proposed back in September, the Conservatives should be working far harder to sign up and engage with such supporters wherever possible.

However it’s expressed, the private sector would be wise to speak up now, rather than trying to buddy up to McDonnell in the hope that he’ll change his very nature and leave them alone. If he ever gets into the Treasury – god forbid – they’ll suddenly find it’s too late, and there are no more tea and biscuits on offer.