Right, just in case anyone else has this problem I'll explain what I did to make it fit (may be useful provided slightly stretched wires are actually safe)
Okay, remove the rail and there will be a horizontal gap that is quite thin (it is between the two side rails at the back), slide the thinner half of the LiFe battery into this gap. Unscrew the gas parts cover and put the thicker half of the battery on the outside of the rail, adjust wires and put the gas parts cover back on. Cut the back corner or the peq box out and wire the battery through the new cut hole. Carefully reassemble the rifle to ensure you don't catch any of the wires.
So far, this actually works, is allowing me to use a 9.9v 1600mah battery with the hornbill rail. Now just to hope there isn't a safety aspect I'm missing...
In the event that someone actually does experience this problem and find I haven't explained it well enough then I'll try uploading some pictures!