Sooooo...the vet put Charcoal to sleep.
He said it was a burst abscess but Charcoal was also really underweight. I could have opted for an operation and antibiotics but with the possibility of it being subcutaneous Mareks I didn't think it was worth putting her through that possibly for nothing. I guess I will now just have to wait and see how the rest of the flock go, they are all different heritage breeds so I guess some will be more prone to Mareks than the others....if that is what the issue is