Had that experience last year, went into loft and found 7 cocks killed and some half eaten.Only possible way of access was the 2 inch gap at the back of the roof and side of the roof.Never found out what it was.It was only in the one section, but all the sections were the same design.The outside of the loft is clad with PVC boards, so whatever it was must have had a good grip in its feet.I moved the remaining 6 cocks from that end and left the dead birds for a few days, doused with of weed killer.It must have come back 2 or 3 times as the carcasses were getting less, until that stopped and I removed what remained, but never found out what it was.I stuffed the air gap around the loft with chicken wire and have had no further bother, but its always in the back of your mind when you go out in the morning, especially when the culprit was never found.