The general rule in Brussels is that what goes on at a party conference stays at the party conference. Rhetorical fireworks from politicians playing to the crowd are regarded as part and parcel of the political cycle, to be safely disregarded as a bit of theatre. But with six months to go until Brexit, the normal rules do not apply. Every cough and spit in the British debate at this stage is attracting the closest attention, and much of it is causing great anxiety in the EU quarter of the Belgian capital.

Jeremy Corbyn was first up, with his leader’s speech in Liverpool. To say it was forensically examined and discussed by EU officials and diplomats does not do it justice. EU officials liked bits of it. They chortled in the European commission at Corbyn’s line that the government envisioned a “Britannia that both rules the waves and waives the rules”. They thoroughly approve of Labour’s policy on negotiating a customs union with the EU, and have had this message passed on to the Labour leader. But the central thrust of Corbyn’s speech worried them deeply.

Corbyn has made it clear he is sticking with his “six tests” as the criteria for judging whether Labour will back the government in the meaningful vote on a deal in parliament. The six tests – not least the one insisting that a deal must offer “the exact same benefits” of single market and customs union membership – are designed to help Labour vote down whatever May brings home.

Certainly Labour members are clear that they would vote down a free trade agreement of a so-called Canada+++ nature (the pluses are increasing in ratio to the desperation in Brussels for it to be accepted). It might cover lots of sectors, from fisheries to aviation, and avoid tariffs, but it would leave Northern Ireland in the EU’s customs territory and do little to protect Britain’s manufacturing base from checks, red tape and worse. The former home secretary Amber Rudd has said 40 Tory MPs would join Labour in the “no” lobby. The EU’s executive senses danger, and it has started disregarding some diplomatic norms to try to mitigate against it.

On Thursday Corbyn was in Brussels for the inauguration of Place Jo Cox in honour of the Labour MP murdered a few days before the 2016 EU referendum. On hearing that Corbyn would be in town and likely to visit Michel Barnier, the commission’s all-powerful secretary-general, Martin Selmayr, known as the “monster of Berlaymont” for his tactical nous and work ethic, let it be known he would like to talk to the Labour leader.

It was a potentially controversial move. EU heads of state and government – acting as something of a leaders’ union – have previously told Barnier, their chief Brexit negotiator, they felt distinctly uncomfortable about him meeting the leader of Theresa May’s opposition. Selmayr’s rendezvous would do little to ease concerns. When the Guardian broke news of the meeting, the commission’s spokesman told reporters there was no meeting of which he was aware. Weasel words that were swiftly shown to be so. The meeting took place. Selmayr laid on the charm, as he is very capable of doing.

Selmayr, who is in charge of no-deal preparations and who will take control of the trade talks after Brexit day on 29 March 2019, wanted to keep open the lines of communication and keep talking. But if Selmayr was the good cop, Barnier, in the next hour-long meeting, proved to be very much the hard-hitting bad cop, willing to crack some heads to get a result. Corbyn was told that his stance made a no-deal Brexit more likely than not, sources in Brussels claimed, although talking to reporters later the Labour leader insisted that no opinion on his party’s position was voiced.

Barnier and his deputy, Sabine Weyand, told Corbyn they believed they might be close to finding agreement with May, but everyone needed to act calmly and responsibly if the withdrawal agreement was to get through parliament. Senior figures in the commission believe Labour could play a key role in de-dramatising the issue of the Irish border. It is likely to have been an unwelcome lecture that exposed just how worried the Eurocrats are, even before the Tories have started their caterwauling in Birmingham.

Asked what they would most like to hear from the Conservative party conference getting under way this weekend, there are two responses from EU officials and diplomats involved in the talks. Initially, they express their frustration. They have no understanding of the political fissures and rivulets that run through the Conservative party, little understanding of the Downing Street strategy, and have almost completely given up on trying to work with May in a coordinated way to stage-manage a result. It just doesn’t seem possible.

That might all sound rather gloomy. But it is the second reaction that should cause the greater concern. Some say they just hope the Conservatives can have a “sensible, rational discussion” in which they accept that keeping Northern Ireland in the EU’s customs territory is the only solution for avoiding a hard border on the island of Ireland, and come to realise that the EU is offering a trade deal like no other. The coming days and weeks are likely to prove a considerable disappointment.