I think the big problem for Labour is they need to pick up 64 seats to win a majority and if you just look at the marginals in England and Wales they would have to win some they lost by sizeable margins. Otherwise beating the Tories in seats is easy, getting a majority seems a lot tougher. The one way I could see them doing it, is make big gains in Scotland and win around 40 seats there as it seems big swings are more common in Scotland than England and Wales and then pick up another 30 seats in England/Wales.
The swing (if there is one) against the incumbent government tends to be bigger in the marginals than overall. Hence the Tories only got a majority of 21 in 1992 for example as opposed to the 70ish they would have achieved on a uniform swing.
