Have had a number of hens being killed by the others A couple every few weeks we pasture them and use large mobile yards so I don't think it is space could the weak broody hens be the victims?

Well, I guess they could. I mean if they are pecking at each others behinds then that is something called molting. It is a seasonal thing.

another view

The most likely place the the hens are being attacked is where they roost and that still could be an overcrowding problem. Ideally you want at least 18 inches of space per bird on the roosting bar.

The broody hens are often the aggressors and will defend the nest vigorously. Check how much light gets into the coop at night, as just a little light when they are roosting will cause picking thereby weakening the victim and then the other hens will continue to harass that bird all day further weakening and killing the hen.