Unlikely to be a bird of prey - unless you've got a golden eagle dropping in for a takeaway. You said they were fairly large hens. But you're in Staffordshire so it
is unlikely.
The buzzard is quite widespread but generally, it will only take up to the size of a small rabbit and then only if it is easy prey. It's quite happy to feed on road kill, rodents, beetles, frogs, earthworms... owls aren't big enough to take a full-sized hen except for (slightly possibly) the European eagle owl and there are thought to be only a few breeding pairs in England so it is very unlikely.
There's always the possibility of an escapee of some variety from a bird of prey centre but realistically, the hens are unlikely to have been taken by a bird of prey.
A five-foot fence is not going to trouble a hungry fox - or a human predator.