Bald chicken!

Martha the chicken started to moult late in December - it seems to have been a slow process, and she's still losing feathers around her head. From what I read it seems that chickens who moult late (she's nearly three and hasn't moulted before) lose their feathers at a slow rate. She's perky, has a red comb and is still laying so I'm not too bothered about her health. But she does have a bare bum, and looks very oven-ready from an inch below the vent to her undercarriage. There don't seem to be signs of new feathers either. I have tried anti-pecking spray (in case the others have been having a go, but I haven't seen any evidence of this), have given her a little cat food as a treat, which has certainly improved the rate of laying, and dusted for parasites. (Not all at once...) A friend at school who is a smallholder said that in her experience some chickens never re-grow these feathers.
Anyone got any advice on this??