This is the second story in the collection by H.R.F Keating called In Kensington Gardens once... . It comes from a 1985 publication called Mrs Craggs: Crimes Cleaned Up, which I really must find. In this story we meet Mrs Craggs whose job is a cleaner. She works in both the Royal Albert Hall, and at private homes. On her way from one to the other, she passes through Kensington Gardens and:
lingered at the Albert Memorial, climbing its many shallow steps and wandering round looking at all its myriad statues.
She begins to see the same three people there, having some sort of meeting.
From the start their talk looked as though it was meant to be secret. But there was seldom anyone else nearby at that hour and somehow the three of them, having seen Mrs Craggs every day, soon discounted her presence and spoke quite loudly.
This 'discounting' leads to the downfall of a murderer. How it happens makes for an interesting and very enjoyable little mystery, all told in only six pages. Mrs Craggs is an observant sleuth, both of physical details and human nature, and this quality allows her to know who the killer is.
The Albert Memorial