So, I've just been back and checked, and its been 16 days since I first noticed that Maeve wasn't herself. There is still no obvious sign of illness, and she is eating and drinking. However, she seems quite half hearted when running for treats, and is generally pretty quiet. This is most out of character.
She pootles about the garden with the others, but retreats to the coop every so often. Sometimes I find her in the nest box, but she's just as likely to be perching. Her crop appears fine, as does her vent. There is no discharge or smell from her head, and I've noticed no unusual droppings in the coop. She hasn't lost weight.
As far as I can tell, there is no bullying (who would dare?). She's just not herself. I've been putting poultry spice and garlic powder in the pellets, in the hope of boosting her a bit. No change, as far as I can see. She has come through her moult now, but is not yet back in lay. She's about a year old.