I know it has been awhile since my last reply but I thought I should update this in case anyone else was interested or has a similar problem. After doing a lot of research I found some literature stating the problem with feather pecking can be them trying to replicate natural foraging behaviors.
Basically because they have a feeder with all the food they could ever need they pig out in the morning get bored by the afternoon and then terrorize each other.
So I decided to try something.
I removed the feeder and every morning I rake the entire run clean scatter pellets, grain, oats, grit and oyster shell throughout the run and then lightly rake it over to cover it.
The difference this has made has been remarkable. The pecking stopped instantly. They spend the day digging for food instead. The hen who had her feathers plucked out as regrown almost all of them! She just has a small bare patch left on her back now which has new feathers currently coming through.
I haven't had any feather pecking in months and they all seem much happier.
Thanks all for your help!