Senior gave Fallon London the heads-up in an all-staff e-mail, which was sent early on Thursday morning, warning the agency of the news that he was involved in the creation of a mini agency group that would align the agency he co-founded with its Publicis Groupe sibling Saatchi & Saatchi.
Senior and Laurence Green were in the US, in meetings with the Fallon president, Pat Fallon, and the Saatchi Worldwide chief executive, Kevin Roberts. The four were trying to establish how the alignment, which Senior will head in the UK and Roberts in the US, will work. The message didn't arrive until 3.00 on Thursday morning, suggesting the meetings had gone on into the night in New York.
Such an e-mail probably raises more questions than it answers. How can one chief executive divide himself between two agencies, both of which are more than a full-time job? And, provided a suitable balance of time and responsibility is struck, how does that chief executive allay client fears that their marketing secrets are safe behind what must appear fragile Chinese walls?
This will be far harder in the UK than it will be in the US, where Fallon and Saatchis occupy different time zones and are radically different beasts-the former dominated by local clients, the latter a Procter & Gamble powerhouse.
Here, though, Fallon and Saatchis bear more similarities, culturally and in their client lists. And they are barely a mile from each other as the crow flies across the most parochial advertising landscape in the world.
The "alignment" between the two agencies that was thrashed out in a series of meetings in New York and Minneapolis last week began life as a suggested merger between the two Publicis Groupe brands.
It's not hard to see why the Publicis Groupe chairman and chief executive, Maurice Lévy, might have looked at this option. In its native US, Fallon has been holed beneath the water so many times over the past two years that many at Publicis are questioning if giving it one more chance is just prolonging its agony. Merging it into the strong Saatchis brand there would solve a number of its existing problems.
But in London, Fallon is rampant. It produces strong creative product, and is trouncing allcomers in the new-business ring with some rock-solid senior management. "It's not as if Fallon's a small boutique agency any more, it's grown up over the past three years," a former employee says. "It's not 75 people clinging on to the vestiges of independence any more."
In comparison, Saatchis' Charlotte Street office has endured a torrid three years. Its creative reputation is patchy at best; it has haemorrhaged accounts and has failed to find a replacement for Lee Daley, who quit his role as the chief executive for a marketing position at Manchester United in February.
Like his staff, Senior is passionately loyal to the agency he co-launched in the UK nine years ago. It's a safe bet that he won't have brooked any dilution of the Fallon brand, particularly when its equity has never been higher in the UK.
His friends argue that if anyone can make an "alignment" of the two brands work, Senior can.
Farah Ramzan Golant, the Abbott Mead Vickers BBDO chief executive, was a graduate trainee with Senior at the now-defunct Burkitt Weinreich Bryant Clients & Company. She says of her friend: "While he has clearly changed over time and become an experienced professional, the things that characterise him have stayed the same: he's focused, forensic and fun.
"The big question is can he take the ferocious thing that they do at Fallon to reignite an institutional player such as Saatchis?" she asks.
And that is one big question. The other is how can one man do two jobs? Can a single chief executive reasonably sit in on a meeting with T-Mobile at Saatchis and then hotfoot it across town to discuss Q4 sales targets with his Orange team at Fallon? These two accounts aren't the only clashes - Fallon has the Volkswagen-owned Skoda marque; Saatchis is still deeply involved with Toyota, despite the steady erosion of that account by CHI & Partners. There would also be friction between SCA Hygiene and Procter & Gamble.
The argument that any network chief executive could find himself in a similarly schizophrenic position holds some water here, but not enough to ease the concerns of conflict-jittery clients.
Neither Publicis Groupe, Fallon, nor Saatchis will be drawn on the mechanics of the alignment. But a senior Publicis source said Senior's first task at Saatchis will be to find a managing director-cum-chief executive to shoulder some of the day-to-day workload, and that Senior will be kept well clear of any accounts where there's the faintest whiff of conflict. As for the question of what would happen should the two shops find themselves going head-to-head on a pitch, the source says this situation would be avoided, and points out that the two agencies have only ever squared up in a handful of cases in the past.
Michael Wall, Senior's long-time colleague, says: "There is some merit in the idea of a collaboration between the two agencies. Achieving it, particularly in the UK, is no stroll in the park, that's why Senior is such a good choice. He's got the ambition, the energy, the maturity to pull it off; the harder it seems, the more he'll relish the opportunity."
Senior is eminently principled and is contractually committed to a number of clients at Fallon. The alignment of the two agencies is an untested model, and another question for its new chief is how much bandwidth is left for him at Saatchis after he has fulfilled his contractual obligations at Fallon.
There's no doubting his skill as a manager and as an account man. Alongside the co-founders of the London office - Laurence Green, Richard Flintham and the now departed Wall and Andy McLeod - Senior has made staff and clients feel as if they are in an iconoclastic independent shop rather than in an agency now 100 per cent owned by a giant advertising network.
After Burkitt Weinreich, Senior moved to DMB&B where he was appointed to the board in 1992, aged 28. His skills were honed at his next agency, Simons Palmer Denton Clemmow Johnson, which he joined as the client services director, and where Carl Johnson took him under his wing and groo-med him for senior management.
When he and Wall were lured out of TBWA\Simons Palmer in 1998 to launch Fallon McElligott in London, their aim was simple: be the premier creative agency in the world, producing extraordinarily effective work for a short list of blue-chip clients. It might have taken seven or eight years to get there, but all five founders can be happy they've lived up to that.
There were many people who questioned if Senior would stay at Fallon after his earn-out with Publicis Groupe finished in June last year. Undoubtedly, he and his partners have done well out of the 40 per cent of the agency they owned, and speculation was rife Senior would variously launch another start-up, head to one of the vacant chief executive positions in London, move to Minneapolis to step into Pat Fallon's shoes, or follow Wall's lead and take some time out in the Swiss Alps and think about what to do next. But while that might have been attractive - Senior is a passionate sportsman - his drive will have spurred him to take on another agency challenge. Making an alignment of Saatchis and Fallon work is certainly that.