He said this interaction "festered in your mind from that moment onwards and that was enough to trigger an attempt to cause really serious bodily harm - as an act effectively of revenge".

This zinging back-and-forth is also in evidence in the film, in which these sisters metaphorically tear each other apart and then try to heal the wounds that have festered.

McCartney said confusion over the break-up had festered because the band's new manager Allen Klein - with whom he refused to align - said he had needed time to tie up some financial loose ends.

The first test of McDonnell’s approach as chief is likely to be MacArthur Park, where crime, gang activity, homelessness and drug use have festered.

"We need to prevent the festering of issues and work better together to identify, fix and understand root cause," he said.