They’ve done it again: the government has survived thanks to the votes of four of Labour’s Brexiteers: Frank Field, Grahame Stringer, John Mann and Kate Hoey broke the party whip to vote against an amendment put by the pro-European Conservatives Stephen Hammond and Nicky Morgan to keep the United Kingdom in a customs union with the European Union after Brexit. (Joining them was Kelvin Hopkins, who is currently suspended from the Labour party following allegations of sexual harassment, but who continues to follow the Labour whip.)

The result was all the more surprising because the government had, just minutes before, suffered its first defeat since 13 December, after MPs voted to bind the government to remain fully aligned with European medical regulation after Brexit. What happened?

Contrary to the repeated meme that “pro-European Conservatives don’t rebel” in both divisions – and indeed in yesterday’s crunch vote on VAT – the number of pro-European Conservative rebels was big enough to overcome the Conservatives’ majority on paper, but not big enough to overcome Eurosceptic Labour rebels going the other way. (Although one Conservative MP, Jeremy Lefroy, voted against the government on medicine but with the government on customs, his vote wouldn’t have been enough to change the outcome.

What changed, and did effect the outcome, was that Field, Stringer, Mann, Hoey and Hopkins voted with the Labour whip on the European Medicines Agency amendment and against it on the customs union. Why? It all comes down to the personalities and policies

Field, Stringer and Hoey are the most committed of Labour’s seven longstanding Labour Leavers. (There are a number of Eurosceptics who have compromised their principles to remain on the Labour frontbench.) They will vote to inconvenience the government but they won’t vote to compromise on the essential underpinnings of Brexit.

They (and Mann) are a little different from the other three Labour Leavers: Dennis Skinner, Ronnie Campbell and Kelvin Hopkins, though Hopkins did vote to leave the customs union on this occasion. The difference is that Field, Stringer, Mann and Hoey are all Eurosceptics from the old Labour right (all but Mann held ministerial office in Tony Blair’s first term)while Campbell, Skinner and Hopkins are all from the old Labour left.

The reason why that matters is that an underrated part of Labour’s whipping operation has been the role played by Jeremy Corbyn himself. Labour’s leader is of course a Eurosceptic by instinct and no-one who knows him well disputes that his heart does not still beat to a Eurosceptic tune. But he wants to defeat the government wherever possible and will tilt in a more pro-European direction and towards a softer Brexit if he has to. Crucially, however, his long years on the Labour left means that he has a credibility when he tells Campbell, Hopkins and Skinner that Brexit is safe with him. (Skinner also played a key role in the votes on the meaningful vote in convincing his fellow Labour Leavers that the issue was not one of stopping Brexit but of defeating the Tories.)

But while Corbyn might share a private belief with Hoey, Stringer, Mann and Field, he doesn’t share a history: so his ability to pull them in with him is limited.

Another important bloc are Labour’s seven outspoken New Leavers: Caroline Flint, Kevan Jones, John Spellar, Yvonne Fovargue, Kevin Barron, Helen Jones, and Laura Smith. These are MPs who campaigned for a Remain vote in 2016 but now believe that only a hard exit will meet the referendum mandate. They will vote to defeat the government provided they think their votes will not be seen as blocking the referendum result. These votes didn’t upset any of these MPs but they are a vital group to watch.

And these are the crucial groups in explaining why 14 Conservative rebels isn’t enough to overcome the government and why, as it stands, any talk of Labour having the numbers – even if they had the will – to save the United Kingdom’s single market membership is just that, talk.