This year’s Peter Mandelson Memorial Dim Sum Supper was if anything an even more sombre affair than last year’s. Last Christmas the participants berated themselves for having predicted the 2015 general election wrongly. This year’s proceedings began with another stern self-admonishment.

As minutes secretary, I was sorry to report that last year we predicted Zac Goldsmith would be mayor of London; the Scottish National Party would advance on its nine-seat majority in the Edinburgh parliament (it lost six seats); the EU referendum, in September, would go for Remain; and Hillary Clinton would prevail over either Ted Cruz or Marco Rubio.

(In my defence, I also record that I refused to predict the American election, reminding colleagues of confident predictions in 1991 that the Democrats were merely choosing which unfortunate would lose to George Bush Sr in 1992, when they chose Hillary’s husband.)

Supreme Court Brexit Challenge Show all 13 1 /13 Supreme Court Brexit Challenge Supreme Court Brexit Challenge People wait to enter the public gallery outside the Supreme Court ahead of the challenge against a court ruling that Theresa May's government requires parliamentary approval to start the process of leaving the European Union, in Parliament Square, central London Reuters Supreme Court Brexit Challenge Gina Miller, co-founder of investment fund SCM Private arrives at the Supreme court in London on the first day of a four-day hearing Getty Supreme Court Brexit Challenge A man waves the EU flag in front of the Supreme Court Getty Supreme Court Brexit Challenge Satirical artist Kaya Mar poses with two of his paintings in front of the Supreme Court Getty Supreme Court Brexit Challenge Pro-Europe protestors dressed as Supreme Court Justices stand outside the Supreme Court ahead of the first day of a hearing into whether Parliament's consent is required before the Brexit process can begin. The eleven Supreme Court Justices will hear the government's appeal, following the High Court's recent decision that only Parliament can trigger Article 50 Getty Supreme Court Brexit Challenge The eleven Supreme Court Justices will hear the government's appeal, following the High Court's recent decision that only Parliament can trigger Article 50 Getty Supreme Court Brexit Challenge Businesswoman Gina Miller arrives at the Supreme Court ahead of the first day of a hearing into whether Parliament's consent is required before the Brexit process can begin Getty Supreme Court Brexit Challenge Attorney General Jeremy Wright arrives at the Supreme Court in London EPA Supreme Court Brexit Challenge Protesters outside the Supreme Court in London, where the Government is appealing against a ruling that the Prime Minister must seek MPs' approval to trigger the process of taking Britain out of the European Union PA wire Supreme Court Brexit Challenge A protesters wearing a judge's wigs and robes stands outside the Supreme Court ahead of the challenge against a court ruling that Theresa May's government requires parliamentary approval to start the process of leaving the European Union, in Parliament Square, central London Reuters Supreme Court Brexit Challenge A protester holds up a placard outside the Supreme Court ahead of the challenge against a court ruling that Theresa May's government requires parliamentary approval to start the process of leaving the European Union, in Parliament Square, central London Reuters Supreme Court Brexit Challenge Pro-Europe protestors dressed as Supreme Court Justices stand outside the Supreme Court Getty Supreme Court Brexit Challenge A man waiting to enter the public gallery waves a European Union flag outside the Supreme Court ahead of the challenge against a court ruling that Theresa May's government requires parliamentary approval to start the process of leaving the European Union, in Parliament Square, central London Reuters

The dim sum event is now as much an established fixture in the British political calendar as the party conferences and the annual row over Philip Davies’s filibustering. It began on 23 December 1998, when I was lunching with a group of friends in Soho and the news came through that Mandelson had resigned from the Cabinet. By coincidence, the same group was lunching just after Christmas two years later, on 24 January 2001, when Mandelson resigned again.

The group has gathered every year since, just before or just after Christmas, and its main business is to predict the political year ahead. Even the previous prime minister followed its deliberations. Before the 2015 election David Cameron told me: “I read your column, the dinner party, the dim sum. How do you think the Lib Dems are going to do? We need to win a lot of seats off them.”

He was right, which was one of the reasons we got it wrong. We should have invited him to this year’s supper, really, but we made our unreliable predictions without him.

We started with the Supreme Court: there was a majority verdict in favour of a majority verdict against the Government in January. So a Bill to invoke Article 50 to leave the EU would have to be passed by Parliament. However, we were unanimous that Article 50 would be triggered by Theresa May’s deadline, the end of March.

After that, we thought that none of the three Brexit ministers would resign in 2017, although there was a dissenting vote for Liam Fox, the International Trade Secretary, and we agreed that the UK would actually leave the EU in 2019, at the end of the two-year negotiating period.

The next business was the French presidency. We were unanimous that Marine Le Pen would not win. This may seem like another complacent view from the politics-as-usual bubble, but the conventional wisdom isn’t always wrong and French politics isn’t like politics anywhere else. If French voters decide that Brexit-Trumpism is an Anglo-Saxon phenomenon, Le Pen might not even make the final two for the run-off. Most of the dim-summers thought Francois Fillon, the conservative, would win, although there was a vote for Emmanuel Macron, the independent centrist.

For the German elections in the autumn of 2017, we struggled to see where the threat to Angela Merkel would come from. The social democrats are her coalition partners and the insurgent Alternative for Germany come up against the nation’s fear of its own history. So we waved her through. She would therefore continue to be the most important person in deciding the terms of our departure from the EU.

After that we were unanimous that the next general election would be in 2020 – no early dash for the polls subverting the Fixed-term Parliaments Act – and that Scotland would not hold a second independence referendum by then.

Then we could put it off no longer. It was time for the aromatic crispy duck. Thus fortified, we proceeded to tackle the hardest challenge. What is going to happen to the Labour Party? We had to start by admitting that we had no idea whether Len McCluskey would succeed in his snap election device to hold onto the leadership of the Unite union in April. If his challenger, Gerard Coyne, the union’s West Midlands Regional Secretary, were to win, that would make Jeremy Corbyn’s position more difficult. But even so it was not obvious when Corbyn would go or, more importantly, who would succeed him.

The problem with this year’s leadership challenge from Owen Smith is that so many of Corbyn’s supporters, even if they admitted to themselves in the dark silence of the small hours that he wasn’t the Messiah, felt that it was unfair. “Let Jeremy fail on his own,” as Gavin Sibthorpe, Corbyn’s own events manager, put it in the Vice documentary.

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The question for us was when that failure would be incontrovertibly evident to enough of what remains of the membership surge. The majority view was that Corbyn would be gone by the time of the 2020 election. The minority insisted that, while it felt as if he must go, “analysis” suggested he would lead the party into the election, which would be an unusually one-sided contest.

The discussion about who might be the next Labour leader bounced inconclusively from Clive Lewis to Lisa Nandy to Keir Starmer and to Angela Rayner, the shadow Education Secretary since July. In the end we concluded that Sadiq Khan, the mayor of London whose term expires on the day of the general election in 2020, might be the best bet, despite the awkward timing.

The debate about who would be the next Conservative leader was no more definitive. Boris Johnson, Philip Hammond and David Davis were all mentioned. The familiar golden ageist lament about the paucity of talent on both sides of the House of Commons was rehearsed.