Well, the vet had a look at her today and guess what - there was NO sign of froth or weeping from her eyes! However there is a very slight wheeze, only just detectable, in her breathing.
She is eating and drinking normally and was just very put out at being in an isolation pen.
Vet's advice is to keep her isolated and under observation for a few days, and if she shows any further sign of infection I can pick up antibiotics from the surgery. Also keep an eye on the rest of the flock. He thought she might have picked up a virus, possibly from wild birds (swallows have arrived, so that's a new factor), and is successfully fighting it off.
So fingers crossed!