I’m not sure I’d even heard of Arron Banks, 22 months ago. He was just some businessman who’d given money to Nigel Farage and whom I’d chanced upon when I started investigating the now-defunct data firm Cambridge Analytica.

Some businessman who has loomed large in my life ever since then. Who has called me hysterical, insane, a lunatic, a mad woman, a conspirator, a loony, a mad cat lady, a nasty piece of work, a criminal, a bully, a mad cat lady, a loony, a tinfoil hat nutter, a hacker, a mad cat lady, a loony, a bitter Remoaner, a lone conspiracy theorist, an enemy of the people.

Who has reported me to the police. Who has a file of information on me that includes my address, telephone number, age and some details about my personal relationships. Who has posted memes and caricatures and, in one instance, a violent threat towards me on social media. And who, last week, was referred to the National Crime Agency.

The Electoral Commission has concluded there were “reasonable grounds” to suspect he had committed multiple criminal offences. It couldn’t determine if the millions that he’d donated to Farage’s campaign were from “impermissible” – or foreign – sources.

It’s a landmark moment. Banks is the single biggest funder, by a long way, of the EU referendum, his millions underwrote Farage’s entire campaign. But this is just one of multiple different inquiries into Banks’s business and political dealings; investigations that are being conducted by at least three different agencies. It’s unclear whether they will be complete – or in some cases even begun – by the time Britain leaves the EU next March.

It’s telling though that the news has come, almost exactly a year on from the moment when Banks’s behaviour seemed to change. Because until last November, he had responded with winning good humour to the reports the Observer published, even though the very first piece we published kicked off two other official investigations: one was by the Electoral Commission, which was referred to the Met police in May; the other was by the Information Commissioner’s Office (ICO) examining the use of data in the Brexit referendum.

But two years ago, it didn’t seem so serious. The Electoral Commission had almost no power. The inquiry by the ICO, which will publish its final report this week, has turned into the biggest data protection investigation ever. But we didn’t know that then and Banks agreed to meet me, just days after article 50 was triggered.

I spent an extraordinary afternoon with him during which he joked about his wife being a Russian spy, told me about his diamond mines, his Belizean diplomatic passport, his appearance in the Panama Papers and my hopeless and doomed efforts to hold him to account. “You’re looking for a smoking gun, but there’s a smoking gun on every table!” he said. “And no one cares. No one cares!” They didn’t. He was right.

Then that seemed to change. In a two-week period, Robert Mueller announced his first indictments in the United States over the 2016 presidential campaign and revealed a number of telling links that ran through the Russian embassy in London. The Electoral Commission announced another investigation into the source of Arron Banks’s finances after a series of news reports by Open Democracy and the Financial Times.

As the Brexit bill struggled through parliament, Banks launched vitriolic social media attacks on 12 MPs including Anna Soubry for being “traitors” and “enemies of the people”. And a week after I revealed that the Russian embassy had written to the Observer calling me a “bad journalist” who had “shown my true colours”, he posted a spoof video of me, beaten up and threatened with a gun, as the Russian national anthem played.

This summer, when journalist Peter Jukes and I were passed emails that detailed a covert relationship with the Russian government including introductions to lucrative gold and diamond deals, he reported us both to two different police forces. Since then, he and his sidekick, Andy Wigmore, posted more or less daily attacks. They say there is worse to come.

Banks and Wigmore and that man who is at the centre of this all, Farage, have attempted what I’ve always thought of as a Top Gear aesthetic. They’ve striven, above all, to be the Good Blokes of British politics. The ones with whom the man in the street wouldn’t mind having a pint.

But the mask has slipped. The appearance of Banks and Wigmore at the committee investigating fake news showed contempt for parliament. In their report, MPs noted Wigmore was a “self-confessed liar”, he and Banks lacked credibility and that they’d lied about their links to Russia. Meanwhile, Leave.EU’s attacks on anyone who dares question the path of true Brexitiness have turned vicious. Damian Collins, the Tory MP who chairs that committee, has become a hate figure to them and a campaign has emerged to unseat him. In Britain, where our national identity rests on our ability not to take things too seriously, Banks has been treated like more of a figure of fun than a serious threat. But it hasn’t felt like that if you’ve been on the receiving end of his attacks. Or for any of the MPs on the committee who Banks has tried to smear. He is using “bully-boy tactics” with a “sinister edge” to “abuse and intimidate”, Collins wrote last week.

This is not how Good Blokes act. It’s not to say Banks is guilty of anything. He is innocent until proved guilty. But nor are journalists and MPs enemies of the people.

In America, the media debate how to report Trump’s statements with an earnestness that is a hallmark of the US press. Our humour, tolerance and ability to take the piss are some of the very best things about being British, things that deserve the epithet “great”. But this isn’t Top Gear gone wrong. It’s our institutions – our press, our parliament – under attack. How we treat Banks, how we report on him, how we deal with the multiple probes into him and the other referendum campaigns is how the future will judge us. It’s not funny – take it from me.

• Carole Cadwalladr is an Observer columnist