She has rung to say she will collect them tomorrow. Giving them to a friend who has other horses and wants to take them or riding. So it has worked out OK. Very relieved it didn't go the way I thought.
For benefit of anyone who might be interested, I did eventually get in contact with RSPCA who advised me that if she wouldn't sign them over or take any other action, then I only needed to inform her in writing that I was going to do X in X days and she then has opportunity to respond. If she doesn't, then I can do what I need to do, and the RSPCA would be happy to take them in those circumstances. The woman I spoke to said she thought she had seen them when she had gone to a neighbours and knows that they are in excellent condition and well looked after. She even uses the same farrier I do.
There would be 'surrender' fees which I would have paid but it wouldn't have been a good situation despite the help from the RSPCA.
Anyway, a happy ending. Nice to have one of those!