After daily cleaning and dressing with zinc sulphate, her feet are now dry and she is sound. My farrier was out a few days ago and commented that what I was doing had worked well and he pared back her sole to give better access.
I'll continue treating to keep on top of it until the seperation has grown out. I don't think it helped that her hoof walls had been trimmed so short she was mostly walking on her soles. As they've grown out far more now, I'm not sure if it's my treatment or the growth that has helped most, but at least the seperation is clean and dry now, with no sign of infection.