Hello Doncro. We have a problem with all of ours eating the cockerel's combs. They peck the scabs off and the cockerels let them -seems to be a preening arrangement. Unfortunately when they get a taste for the blood they want more. The worst case was two wyandottes, separated for the offence for 3 months, introduced to a new cockerel and in 4 hours they ate half his large comb. The two hens were culled.
So what i am saying is there may be no cure for it and permanent separation is necessary. The hens can never have a cockerel. We have tried Stockholm Tar on the injuries. Like a thick version of the Net tex spray, but a tiny fraction of the price. We stir it well and then apply it with a cotton bud. Doesn't wash off but needs to be reapplied every few days. It has a limited effect though because shortly after stopping the applications one always starts again. We have found the only successful treatment is far more space in the runs as boredom appears to be a major factor. Since letting ours out earlier the problem has reduced considerably and I am now building extensions to the runs they say in when we go out.