I hate this whole, I was happy when I bought it and I have lived with it for 3 years but now that I know I can get an extra so many inches of land bit 
Deal is this, a boundary dispute just makes solicitors richer, they aren't the ones who have to live next to the neighbours you may well piss off.
You wanted the land at the front and the builder has agreed to that so why do you now want what is on the other side of the fence? Trust me, it can turn your life into a living hell. Have a look at the forums on [www.gardenlaw.co.uk]] to see how bad it can get and how long it can go on for.
Put the boot on the other foot, how would you feel if your neighbour said, hey it turns out you have 6ft wide strip of my garden on your side of the fence and I want it back? Even though I have been happy for years, I have just discovered I can have it.
A boundary is an imaginary line between two properties, a land registry plan is 1:1250? So that red line is so flipping wide on the ground, it isn't the width of a fence panel of a few inches.
Seriously, as someone whose best mate went through this herself, her biggest regret is not saying where do I see this in 1 years time, do I think my neighbours will be happy with the fence move. What form of retaliation may they take.