My final message to the polite but adamant eBay seller.

I have had a reply from Simon Bell who knew & worked with Dusty for years as both friend and backing singer. He says that it's 100% NOT Dusty and that is good enough for me and the rest of her fans. The photo is around '68/'69 by which time Dusty had discarded the wild backcombed hairstyle which now was much slicker looking. I know that you won't change your mind and that's fine. We will agree to differ.