A consensus is emerging in the British press about the German elections: Merkel is 100% certain to get her third term in power, making the most important general election in Europe this year also the most boring. Commentators are already casting their gaze beyond 22 September, detecting a pro-British, eurosceptic "change of tone" in Angela Merkel's EU policy and hailing a new "Anglo-German alliance".

Not so fast. Merkel's victory looks highly likely, and Germany's chancellor will certainly be crucial in deciding the fate of David Cameron's renegotiation strategy. But few of the latest analyses seem to factor in how crucial a role the CDU's coalition partner is going to play. Unlike in the current British coalition government, in Germany the junior partner is usually rewarded with the foreign ministry. And the latest polls indicate that this would not taken by the liberal FDP (arguably the closest equivalent you can find in Germany to the modern Tory party), but the Social Democrats (SPD).

Earlier this week, Die Welt reported that behind the scenes the SPD had given up on forming a coalition with the Greens and its leaders were already frantically jockeying for ministerial posts. So who could potentially end up on William Hague's speed dial?

It certainly won't be the SPD's chancellor candidate, Peer Steinbrück, who has ruled out taking a ministerial post in a Merkel-led government. A much stronger contender is its chairman, Sigmar Gabriel, who has been busy weaving a net of global contacts over the past few years: he was one of the architects of the "Progressive Alliance" of European centre-left parties, a rival organisation to Socialist International.

Many think that the most likely foreign minister in a new grand coalition would be the same man who occupied the post during Merkel's first term: Frank-Walter Steinmeier, who is currently the formal leader of the opposition. Cameron should take note. Steinmeier not only has a history of scuppering Merkel's foreign policy plans from 2005 to 2009, but is also considered one of the Social Democrats with the closest links to the French Socialist party (Stephan Steinlein, his francophile chief of staff, was East Germany's last ambassador to Paris).

In December 2011, Steinmeier said he expected Great Britain to leave the EU: "I fear the decisive step for Great Britain's exit has already been made. If the regular meetings take the form of a Europe of 26 without Britain, then a process of alienation will become inevitable and irreversible."

It's true, Europe's future – and Britain's role in it – will be decided in the chancellory, not the foreign ministry. But the latter may well become more of an obstacle for the former than it is now. For most Social Democrats, the lesson of the 2005-2009 grand coalition was that they did all the hard work and the CDU took all the credit: they're unlikely to keep quiet if they disagree with Merkel's new UK-friendly course this time around.