Hi Mauve, I don't know if you got your problem sorted, but I had similar issues and a bit stuck as to what to do as the anti-pecking spray just made then look all sticky and did not help as they still plucked feathers from the back area.
After doing some internet searching, I came across some research that looked at the quality of chicken food and the linking to feather pecking. It stated that with the introduction of high quality food, hens need to scratch less to get the required amount of nutrients, and so end up pecking each other. It went further to 'dilute' the food, causing the hens to scratch more to get the same level of nutrients, which they did, with a corresponding drop in feather pecking.
So I have:
*put more layers on the ground so they have to scratch for it (but not too much to encourage rats)
*made a poultry saddle to protect her back while the feathers grow back. You can buy these from many sources, but I've found it very easy to make one using the instructions from this site.
http://backtobasicliving.com/blog/make-a-chicken-saddle/*Hang greens in a small wire basket when they are in the run to help limit boredom and encourage pecking on anything other than each other.
I did try one of those pecking blocks, but they destroyed it in two days, and there are only two of them! Ended putting that into the hanging wire basket thingy.
Hope this helps.
Best wishes
Christine