No idea, but it would have been a huge gamble for Brown. If he really wanted to stay in power, late 2008/early 2009 would have been his best possibility. He had to pray for voters to believe he was the only one able to solve the crisis and that he needed a clear mandate for that.
But even then, I wouldn't bet on his chances.
If he went into 2009 in more of election mode, he could've done it. Cameron had a bit've a rabbit-in-the-headlights moment compared to Brown on the economy when things went sour when the markets crashed.
Would've been horrific for all 3 if
The Telegraph had leaked the initial stories of the expenses scandel a few months earlier though...