Supposing London suffered a significant Islamic terrorist attack, something like July 7th 2005 or even worse, and this led to a renewal or enlargement of the scope of the 'War against Terror'?
Brown needs some sort of theme around which the various factions of MPs and the party supporters can unite. It isn't going to be the education, welfare stuff or NHS because all that has been done to death and they have completely run out of ideas. But a severe external attack would have the capacity to draw it all together.
Brown could pose as a competent, Churchillian sort of figure - 'the saviour of the hour' and announce a new focus of policies, the security agenda, with a crackdown on illegal workers and such as the scapegoats. New Labour haven't really tried the security agenda. If the USA attacks Iran later this year, he could also join in this and try to pretend his policy was something significant on the world stage.
Such a change in the political climate would gradually alter the opinion polls in his favour, and it might just be enough to swing it in his direction, by the General Election, given the way that they have re-gerrymandered constituency boundaries.