Thanks for the comments.
Stamper, I was limited in my movements at that point - it was a tense situation. We had been following her for about twenty minutes and then she saw the impala. She stood like that for about six or seven minutes
before making her move (and disappeared out of sight). It took us a few seconds to start the vehicle up and get going, by which time she had made the kill - this is the first usable picture I got from the moving Land Rover.
An amazing experience, especially seeing the mother's reaction. She tried to follow the leopard while "barking" and snorting at her - trying to see what had happened to her lamb... that is the way nature works, I suppose.