In reply to ACollins:
Yes, it's fixable. That happened to me in Cairngorms few summers ago - I couldn't fix it in field conditions, so visited a shop in Aviemore with no luck. A girl in Cotswold Outdoors fixed it in couple of seconds without damaging the rail! I am not that handy, I suppose, so I came up with another solution. I just broke/flatten the rail with a screwdriver (it's some sort of flexible plastic pipe going) at the bottom which makes the rail narrower. That way it is really easy to put it back and given you don't slide the buckle down, you are fine! During normal use, I have never had problems it sliding down, but that's why I break the rail at the bottom instead of the top.
To be honest, I really hate these rails. Old style webbing and some sort of sliding buckle is way much better, but I can see why manufacturers move to something that looks better.