[PhilJ]

Add a trendy 'sticky' header in 2 quick steps (v3)...

Demo (header style #3, boxed header - for other styles you'll need to tweak the CSS below).

1) In skin/reboot/js/reboot.js

At the end insert...

// STICKY HEADER
//-----------------------

$(window).scroll(function() {
  if ($(document).scrollTop() > 150 && window.innerWidth >= 992) {
    $('.container_header').addClass('shrink');
  } else {
    $('.container_header').removeClass('shrink');
  }
});

2) In skin/reboot/css/reboot.css

At the end insert...

/* STICKY HEADER
-----------------------------*/

.container_header { transition: all 0.5s ease; -webkit-transition: all 0.5s ease; position: relative; }
.container_header.shrink { width: 1170px !important; position: fixed; top: 0; padding: 0px; background: rgba(0,0,0,0.7); z-index: 9999; }
.container_header.shrink img.header_logo { height: 65px; width: auto; }
.container_header.shrink .header_padding_top_search { padding: 10px !important; }
.container_header.shrink .header_padding_top_buttons { padding: 15px !important; }
@media only screen and (max-width: 1199px) {
    .container_header.shrink { width: 970px !important; }
}
@media only screen and (max-width: 991px) {
    .container_header.shrink { width: auto; position: relative; padding: auto; background: inherit; }
}

Note, appears in 992px wide and up only.
Looks better if you have a transparent PNG logo image.