The value of a vote

I live in an ultra-marginal seat. The tories won by 0.4% of the vote last time, which means I have become very popular all of a sudden. Today I received personalised letters from Labour and Conservative, urging me to vote for them. The Conservative one was entirely negative, completely obsessed with stopping a Labour/SNP alliance. The way they portray the SNP seems almost fanatical. At least the Labour one focused on what they would do.
Our electoral system means that my vote is worth a lot to the parties, it could make the difference between gaining a seat or not. But just down the road in a safe seat, it means almost nothing. Odd old system really.