Do you know if her adoptive father left a will? If there was such a document it may give a clue - perhaps a married name or location at the time it was drawn up. He died the year after the last year Ancestry has the probate calendars, but they are available at some record and probate offices. There is at least one RootsChatter who does lookups at the main office in London from time to time, so if you're not able to do so you could ask in the Common Room or just take a chance and place an order via the postal search service.