Explanations

It's probably just me having missed the point for the last couple of months, but reading Decca Aitkenhead's article about Hartlepool (found via Perfect), I've found an interesting reason why Labour called it for this Thursday (emphasis added):

The Conservatives finished second here in 2001. Two leaders later, even that lies beyond the hopes of all but the party's wildest optimists, but if they cannot hold on to third, Michael Howard will be facing a mutinous conference, and a crisis that could spiral beyond his control.

To be really cynical, one could argue that for Labour winning or losing Hartlepool isn't as important as whether the Conservatives come 3rd or 4th. Sure, a loss would be embarassing, and increase the long-term risk of the Liberal Democrats, but in the short term (the period to the next election) the Tories are the biggest threat, and if there last pre-election conference can be overshadowed by a disastrous 4th place in Hartlepool, there'll probably be smiles all round in Downing Street.