A gig in Romford for Boris and his roadies

London's mayor goes walkabout, but runs into his biographer Sonia Purnell

Boris Johnson, Mayor of London, is notoriously difficult for Londoners to get close to. He is the most highly protected and stage-managed politician in Britain, and his minders allow access to only the most reliably sympathetic or adulatory journalists. As the author of an unauthorised biography, Just Boris, it was therefore with some concern that I set off, unannounced, to join him on the campaign trail in Romford, east London, yesterday. It was only with what I felt to be pretty gritty determination – and the presence of a reassuringly burly photographer – that I managed to sidle up to him at all. His minders are known for their ferocious handling of dissenters, and I was apprehensive of being "dealt with" myself.

The last time I saw Boris Johnson was when I bumped into him in a hotel corridor at the Conservative Party conference last autumn. I can't say who was the more surprised, but our brief conversation was in any case curtailed by a mob of journalists descending on us before I could ask anything remotely probing.

So, yesterday, I was looking forward to meeting up again, in Romford, for a longer chat about the more serious aspects of his bid to be re-elected as mayor. The intent look on his face as he hissed to his wife, Marina, that "Sonia Purnell is with us", suggested he was not quite so eager. I quickly offered my hand and, in front of a growing crowd, to his credit, he grabbed it warmly before declaring with his trademark wit: "Sonia has benefited from my employment policies. She now has a full-time job... bashing me!" Only later did he admit in a quiet moment that "people" had told him that, although critical in parts, Just Boris was "actually rather good". But this welcome "olive branch" (boy, has he studied the charmer's handbook) did not extend as far as answering my questions.

Trailed by a bevy of minders of varying degrees of friendliness, he made quick progress from the station to Romford's historic market – yesterday decked out with hundreds of flags. Boris himself was sporting a suspiciously clean tie, with St George's crosses on it, and a jacket that suggested it had spent the night on a floor. In a green leather coat trimmed with fur, Marina, her usual neat self, looked older and graver than previously.

To begin with, his entourage outnumbered voters, but Boris quickly homed in on the rather startled-looking residents of Romford, and especially its women. Most were charmed; some blushed at being in the presence of a sex god; many asked to be photographed with him. One star-struck couple informed me that "This sort of thing doesn't happen in Birmingham". He smiled obligingly, although he couldn't quite shake off an air of fatigue and faint discomfort. The handing-out of a leaflet was the signal to move on. Occasionally, though, a woman would clasp him and unburden herself. Only the pursing of the Boris lip and the working of his cheek muscles would betray his impatience. "Oh, she's had him too long," snapped an aide.

It was not long before I was the one provoking the cheek muscles. I wanted to put to him so many questions that remain unanswered, and even unasked. It is fair to say that he – and his press chief, Sam Lyons – had different ideas. Questions about the now notorious anti-cyclist comments of a major donor (the chairman of cab firm Addison Lee) were batted away with a classic Borissian "I don't know anything about that". A request for his views on the effects on poorer Londoners of George Osborne's Budget were met with a nonsensical "We're fighting on the issues". Pressed on former Conservative Cabinet minister Michael Portillo's refusal to back his candidacy, Boris somersaulted into saying this was an endorsement of his refusal to back a third runway at Heathrow. At one stage, he rounded on me to say: "Look, the really important issue in this election is the 24-hour travel pass." That's funny, though, because that does not even appear in his nine-point plan for "a Greater London".

By now, the general air of carefully staged Boris adulation was disturbed only by a BNP supporter on a megaphone asking why the Mayor was letting in asylum seekers and digging up roads. A brass band hastily struck up and drowned him out – even the threat of rain was somehow successfully "dealt" with. Indeed, anyone intent on questioning Boris too intently was bundled away, as was a man advising the Mayor to "stop chasing the money men, Boris!".

On went the procession, with Boris, perhaps unthinkingly, posing by a stall selling patriotic babywear in front of the fine old church of St Edward the Confessor. "Why do you think no one mentions your personal life any more?" I asked the Mayor quietly, at which point a particularly determined minder barged in, sending me firmly out of Boris's orbit. Absurdly, he later squared right up to me, as if preparing for a fight, to pronounce: "You can't ask him questions. You've no right to be here."

And so, banished from the parade, I watched Boris disappear back towards the station with Marina holding on to his arm.

Romford might have been royally entertained for an hour or so; but I do not think that any of us is any the wiser as to what Boris stands for, apart from himself.

Sonia Purnell is the author of 'Just Boris: A Tale of Blond Ambition'