the two most high-profile conservatives joined forces to question the legitimacy of the snp 's plan to try to put ed miliband in downing street , even if labour wins fewer seats at the general election .
the london mayor , deployed to inject more passion into the tory campaign , claimed it would mean truckfuls of taxpayers ' dosh growling up the m1 to scotland ' .
the first time since 1924 that a second-placed party had formed a government .