jQuery.fn.centre = function ()
{ 
    this.css("position","absolute"); 
    this.css("top", (($(window).height() - this.outerHeight()) / 2) + $(window).scrollTop() + "px"); 
    this.css("left", (($(window).width() - this.outerWidth()) / 2) + $(window).scrollLeft() + "px"); 
    return this; 
}