In the end I contacted FamilySearch and a couple of days later received a message to say they'd fixed it. They also sent me the link to a tutorial on how to edit relationships (which I didn't need, because I do know how to do it, but the site wasn't working properly)
So, using MY log-in they made changes to several people which weren't changes I agree with! They probably thought they were being helpful.
I do hope this doesn't start a storm in a teacup again with someone who is about a 5th cousin to me but not related other than my marriage to this ancestor of mine. In October and November last year he made numerous changes to this ancestor which I repeatedly edited back, because he really believed she was the daughter of her grandparents who were in their fifties and sixties when she was born, whereas we know she was the daughter of their teenage daughter.
I have no idea why they registered her birth as their daughter, but in the end she was mostly raised by her aunt and was well into her childhood when she started to live with her mother. Her marriage notice in the paper says she is Mrs So-and-So's daughter. her mother, Mrs So-and-So had only married a short time earlier to a widower she'd been living with for quite a while - she's on the electoral roll as his wife a couple of years before they actually married).
Some of the changes have also been made by FamilySearch.
