The "north-east" is a region or area of the country. "To take for granted" means to assume that something will (always) be true. One can say, "He takes his health for granted. He thinks that he will never be sick, so he eats whatever he wants." In your context, Mr. Miliband cannot assume that he will win that area of the country. He should be concerned about that region, because he might lose it in the election.