An assassination of her husband, preferably before Iraq, would make a good PoD. There was a recent case of a Labour MP in Sheffield dying and his wife winning his seat in the by election, perhaps something similar takes place, seeing as Cherie did run in North Thanet in 1983, and she becomes an MP either for Sedgefield at the by election or for some other constituency at the next election. The sympathy for her would allow her marriage to an ex PM to work for rather than against her.
Brown becomes PM first and wins Labour a third term, and eventually promotes her to the cabinet, but as the financial crisis hits, his popularity plummets. The fact he has been PM for far longer than he was in OTL means that plots to replace him are more successful. Cherie Blair, who is extremely popular with the public due to events surrounding her husband, is seen as the only candidate who can win Labour the next election, and she is chosen to succeed Brown without much serious opposition to become Labour leader and PM to boot.
Her popularity might be just about enough to either allow Labour to emerge from the 2010 election as the largest party, or at least large enough to make a deal with the Lib Dems workable, and she is likely to continue for another five years.
