Hi Waterman it looks like it has a case of feather mites that is causing the loss of feather on its head.
If it is fluffed up and lethargic it could also have Trichomonosis have a read here www.bto.org/.../trichomonosis
You may have to watch it around food and water as it can pass the disease on, if it was me I would be shooing it away when I saw it just in case.
Thanks Alan, we have emptied our feeder and water bowl to minimise impact to others. Will keep chasing it away. Intention is to leave a week before feeders go out again after giving them all a good clean. It is fluffed up and struggling to swallow.