Thank you for getting back to me so speedily. I've tried and..............failed.
Went to "My Cemeteries" and opened the St Mary at Finchley cemetery map from "Cemeteries I have added".
The pencil icon appears against the Cemetery Information section but when I go to the map itself I'm not able to move the boundary line. The hand icon on the map lets me move the entire map around but doesn't change to an arrow and let me move the boundary points.
Going to the "Cemetery Map" section, where there is no pencil icon, I get the same result. Can move the entire map but not the boundary line.
Could it be something to do with using Chrome on a Mac? Does that change/limit the functionality of the keys and mouse?
I did create the initial boundary on my Mac with Chrome so looks like I'm missing the plot somewhere. I'll keep trying different options and see what happens.