loook at this one I just bought for my round the world trip, it did cost �150 but really well made. After talking to many travellers they all told me to get a pack that can be wheeled as well as carried on your back (as most of the time its possiable to wheel it and save your back. This one is front opening too...
http://shop.mandarinaduck.com/jsp/home.jsp?noc ash=1157625189550
there is a mandarina shop in london or you can order it online, let me know what you think??