UNDERCURRENT James Cusick

There isn't supposed to be a Tory mayor of London, just like there isn't supposed to be an SNP administration in Holyrood. Listen to the home secretary, Jacqui Smith, and her aggrieved tone on how Boris Johnson choreographed the departure of the head of the Metropolitan Police, Sir Ian Blair. The office of the commissioner, said Smith, was an indispensable part of the good policing of London, a job central to "maintaining the security of the country as a whole." If Smith is confused that her authority as home secretary has been over-ridden, she should ask which party made the constitutional changes that allowed Johnson to organise the coup, a legal one, that forced Sir Ian's exit.

The ousting of Blair from the Met was, if you believe senior political figures at the Home Office, irresponsible. Smith's support for Blair sounded almost politically "quid pro quo", as though an invisible, enforceable contract between Labour and their top cop existed, but had now been abused by Johnson. "Responsible politicians," said Smith, had the job of supporting those like Blair, who carry "heavy operational burdens". Labour had honoured the contract; Boris the Irresponsible hadn't. Whitehall's lawyers say they've just witnessed a "black day for the constitution."

Why the gloom? One source quoted in The Guardian on Friday said Johnson had wanted a change in who led the Met "and that's not the way it should happen." So how was it supposed to happen?

Smith seemed prepared to wait for the verdicts of the three issues that sur-rounded Blair. The inquest into the shooting of Jean Charles de Menezes is ongoing; the race discrimination case brought by assistant commissioner Tarique Ghaffur is to be worked through; and an investigation into £3 million of Met contracts awarded to a close friend of Blair's is yet to be concluded.

The Home Office was patient, if only because Blair had been loyal and backed the government over the introduction of the 42 days anti-terror detention law.

But Johnson wasn't patient and not being part of Brown's government, doesn't have to be. As the new head of the Metropolitan Police Authority, Johnson set about doing what he'd promised during his mayoral campaign. If Blair was surprised at being given the boot, he shouldn't have been.

What was supposed to happen was this: Ken Livingstone was supposed to win, just as Jack McConnell wasn't supposed to be heading to Malawi. Seamless, uninterrupted, uniform government was supposed to happen. Inside the political architecture of devolution, there wasn't supposed to be outside dissenting voices, let alone dissenting power. But there is.