Attack

It sounds like a fox attack to me Pete.  Really sorry you lost your hens this way.  It must have been really upsetting to find that 

Yes a fox would I'm afraid     Not much consolation I know, but they cannot really help themselves when presented with so much food.  They kill everything they can and then go on a mad hide and bury mission with as much as they can.  They often end up running out of time and leaving hens strewn everywhere.

It might be worth reinforcing the bottom sections of your run with weld mesh, as foxes will keep coming back once they have had a success.

I'm sorry to be the bearer of bad news, but it really does sound like a fox attack now.

This must have been so horrible for you to discover, my sympathies are with you.
When i first had chickens & that was long before back garden chickens was as popular as it is now(this time round) I could only find anything out about keeping them in books from the library. I read then & it has stuck with me, a fox just needs to get lucky once, a Chicken keeper has to get lucky every night (and day with urban foxes).
I think the fox will check every night & maybe more than once as it does it’s rounds. Male urine is supposed to deter them but personally I wouldn’t rely on that. I’d take this opportunity to make a secure run with weld mesh as Newshoot suggests.

Foxes will be cubbing now so the parents will be bolder with a family to feed .