I agree with you Lucifer. There is a 'but' though.
Blair will go before the next Gen Election. What the Tories I suspect want to avoid is for Brown to come into the election as PM, still within his honeymoon period.
And that means Brown taking over at least a year before the election, preferably before that.
If they could get Prescott's scalp, that would, according to most political pundits, precipitate a leadership contest too, with Brown becoming PM before or around the time of the Labour conference.
The longer Brown is the PM, the more likely the electorate, notably middle England, will say, 'No bloody way'.