In order to craft a response to Brexit, the shadow chancellor John McDonnell has put some of his personal ideological bugbears in the deep freeze.

One week on from the moment Britain realised it was out of the European Union, McDonnell pledged to fight for the single market, and in particular the right of big banks to passport services.

He told a packed press briefing: “Whilst there is a need for fundamental reform of the City we shouldn’t allow it to simply sink beneath the waves.”

He laid out the principles that Labour would go into a deal with – defending workers’ rights, protecting EU residents in the UK and vice versa, continuity of a single market, a seat at the European Investment Bank and EU passporting rights for the UK’s financial services.

With more obvious passion, he condemned the “shocking and disappointing” rise in hate crime and also pledged to “never to vote for an EU deal” that failed to protect EU citizens living in the UK.

As for anti-immigration sentiment, he argued this was a symptom of a wider economic inequality. Crucially, though, he has acknowledged free movement of labour will come to an end.

He urged party colleagues to rally to the cause: "When the Tories are in such disarray, this is a golden opportunity for us."

But for all McDonnell’s determination to get his foot in the door of Brexit negotiations, it was hard to shake the feeling he was rearranging the deckchairs on a slowly tilting ship.

Unfortunately the deal, like almost everything in the Brexit aftermath, is as unpredictable as the high seas.

First, there is the leadership challenge. Despite the lull following the vote of no confidence in Jeremy Corbyn, his first mate admitted that a leadership challenge could still be imminent.

He said: “If there is to be a challenge to Jeremy Corbyn, that will emerge I suspect over the next few days.”

McDonnell repeated he will never stand – “full stop”. He pledged to chair what he described as Corbyn’s automatic place on a re-election ticket.

Should the rebel Labour MPs fail to capsize the Corbyn leadership, though, it will still be tough for a half-abandoned shadow cabinet to have its voice heard, whether in Parliament or on the international stage.

To bursts of applause from supporters, McDonnell praised the “heroes and heroines” who had stepped up to fill their colleagues’ empty chairs.

But he ended with a plea: “We’ll cover all the bases, but wouldn’t it be better if people came back and worked with us?”

Finally, only hinted at during McDonnell's briefing, there are the EU negotiators themselves. When asked whether he would vote against Brexit if the final deal contained none of these demands, McDonnell said: “We have to respect the decision that was made. Otherwise we undermine all confidence in the democratic process.”

At a time when both main political parties are in turmoil, the embattled shadow chancellor is astute to chart a course for the negotiations on the horizon. But his chances of getting there could be scuppered within days.