It's an ICS short KAC QD one. Had to hunt high and low for it but found one on Ehobby Asia in the end. There's a couple of different short KAC QD ones out there but being a bit anal about getting the right one it had to be the ICS one as it has 9 holes along the length of the cover. Weighs a crapping ton though. The barrel extends through it so I've managed to keep it at 300mm.
Detail whore that I am it is winding me up that the rail is half an inch too long, it's the wrong EO Tech and I don't have the right rear sling mount but screw it, it's cost me a small fortune in parts already! Which of course means there will most likely be some M4 front end bits going up for sale in the near future!