Hi guys, sorry for the long wait for a reply. The client has been back on to me, and I have been playing around rebuilding the child theme on my localhost.
Unfortunately due to terrible compatability from Namesco, I cannot load it onto a demo server.
Everything is working bar the header. It turns out I had modified the AVIA.JS file for the child theme, but I’m NOT a coder, and I’m struggling to see how I can implement this in the current version. Please see below for the code I used in the child…

/* Navigation mods to lower links in header. Follwing function needs to replace existing function in AVIA.JS */
function avia_header_size()
    {
        var win	            = $(window),
            header          = $('.fixed_header #header'),
            //logo			= header.find('.logo img'),
            logo            = $('#header_main .container .logo img, #header_main .container .logo a'),
            elements        = $('#header_main .container, .main_menu ul:first-child > li > a:not(.avia_mega_div a)'),
            el_height       = $(elements).filter(':first').height(),
		// START Reposition Menu - find height of .main_menu
			top_height		= $('.main_menu').height(),
		// END Reposition Menu
            isMobile        = 'ontouchstart' in document.documentElement,
            scroll_top		= $('#scroll-top-link'),
            set_height      = function()
            {
		// START Reposition Menu - include new variable topH
                var st = win.scrollTop(), newH = 0, topH = 0;
		// END Reposition Menu
                if(st < el_height/2)
                {
                    newH = el_height - st;
		// START Reposition Menu - Set topH at max
					topH = 175 - st;
		// END Reposition Menu
                    header.removeClass('header-scrolled');
                }
                else
                {
                    newH = el_height/2;
		// START Reposition Menu - Set topH at min
					topH = 75;
		// END Reposition Menu
                    header.addClass('header-scrolled');
                }

                elements.css({'height': newH + 'px', 'lineHeight': newH + 'px'});
                logo.css({'maxHeight': newH + 'px'});
		// START Reposition Menu - Assign topH to the CSS class
				$('.main_menu').css({top: topH + 'px'});
		// END Reposition Menu
            }

        	if(!header.length) return false;

            if(isMobile)
            {
                return false;
            }

            win.scroll(set_height);
            set_height();
    }

How could this be achieved in the current skin? Many thanks!