I found it about 2 years ago - someone had a copy for £120.....but have since forgotten the name.
I would have read it in the v late 1970s. It was a girls novel - paperback - the front cover was a close up of a girls face and she was wearing a gold crown. I don’t think that the novel was period - and can’t remember the significance of the crown. But I would love to find it and see what it was that hooked the 10 year old me. Sorry so vague ......