Thank you all for your replies. I separated them again last night and she got attacked again as soon as they were let out this morning and is currently hiding under a bush. Our 3 runners are not related, but hatched together in our incubator, have been happy together for their 3 years and willingly mating. Fidget, the drake is silver, the bullied girl, Digby, is white and the other 2 black and brown. I've checked Digby all over and she seems fine and healthy - looking beautiful with her new set of feathers. I've always seen her as 'top' duck, with the others looking up to her and following her, but she has never been mean to others. This sudden change is astonishing!
The only other change I can think of is that we also have eight 15 week old chickens (4 male, 4 female) who are 'coming of age'. Maybe Fidget senses this and for some reason thinks Digby is now 'one of them'.
I'll watch carefully over the next few weeks and hope things settle down. I hope I don't have to rehome either of them. Thank you again for your fast replies from US. I'm in Wales, UK.