Bootstrap scrollspy offset on a fixed navbar does not work

I have created and exact fiddle of my problem here for testing: http://jsfiddle.net/aVBUy/7/

The issue is, when I click on navbar items, I have a script which scrolls to the element id. And I am using scrollspy to highlight the nav elements when page is in correct position. However the scrollspy is only changing the active state when it hits the top of the browser/device. Because my navbar is fixed I need an offset applied to scrollspy to offset by 51px (navbar height).

I've tried everything and I can't get it to work. Please check my fiddle and see if you can find the where I'm going wrong, would help me so much.

Here's my minimised code...

HTML

<div class="navbar navbar-inverse navbar-fixed-top">
    <div class="navbar-inner">
        <div class="container">
            <a class="btn btn-navbar" data-toggle="collapse" data-target=".nav-collapse">
                <span class="icon-bar"></span>
                <span class="icon-bar"></span>
                <span class="icon-bar"></span>
            </a>
            <a class="brand" href="#"><img src="img/logo.gif" alt="" /></a>
            <div class="nav-collapse collapse">
                <ul class="nav">
                    <li><a href="#welcome" data-scroll="#welcome">Welcome</a></li>
                    <li><a href="#about" data-scroll="#about">About</a></li>
                    <li><a href="#route" data-scroll="#route">The Route</a></li>
                    <li><a href="#bike" data-scroll="#bike">The Bike</a></li>
                </ul>
            </div>
        </div>
    </div>
</div>
<div class="container">
    <div id="welcome" class="row-fluid">
        <div class