It began in the heart of Brussels and ended over a sandwich lunch in No 10 on Thursday.

But the story of Peter Mandelson’s surprising return to government at the invitation of his one-time arch enemy is full of strange twists and turns, with a sports fanatic, a newly created baroness and a former advertising executive all playing important walk-on parts.

A long period of non-communication between two of the founders of new Labour ended in May 2007, a month before Gordon Brown succeeded Tony Blair.

Mr Brown, preparing himself for government, spoke to Britain’s trade commissioner in Brussels. They talked about trade and very little else. Mr Mandelson wished the man who had once been a close friend well, and that was that. They agreed to talk again after Mr Brown became Prime Minister and duly did so. But again, the subject was trade exclusively, although there was an almost instant meeting of minds on the big issues facing them, the lack of progress in the world trade talks and the growing impact of globalisation.

There were occasional telephone conversations between the two over the state of the Doha round discussions but although the ice had been broken, the coolness remained.

That was to change dramatically in February this year after a political period which had seen Mr Brown’s fortunes soar and slump. His decision not to go for an early election wiped out the image of decisiveness that Mr Brown had established in his early months.

It was in the heart of the EU’s administrative district near the Schumann roundabout in central Brussels that the seeds of an unlikely political reconciliation were sown. Mr Brown went to the city on his first official visit to the EU anxious to repair the damage created by the chaos over his late arrival for the signing of the Lisbon treaty in December.

It was in the office of Kim Darroch, Britain’s ambassador to Brussels, at the UK European headquarters in Avenue d’Auderghem that Mr Brown and Mr Mandelson sat down together for an hour, their longest conversation for many years.

The first 20 minutes was about trade and then they turned to the British political scene, and the troubles facing Labour.

According to insiders they spoke frankly about their past troubles, without any sense of acrimony. As two of three key architects of the new Labour project they had in the past spent days and weeks speaking nothing but politics, and here they were suddenly doing it again. According to friends they both found it easier than expected. There was a broad agreement that whatever had happened in the past they should try to carry on working and talking together. It was a classic clear-the-air meeting.

Thereafter, Mr Brown encouraged contacts between Stewart Wood, who doubles up in No 10 as an adviser on sport, feeding Mr Brown’s passion for the subject, and European and international affairs.

Mr Wood, a former politics lecturer at Magdalen College, Oxford, already knew Mr Mandelson from their work on the Policy Network think-tank.

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