Information given is extracted from family records. The headstone itself has been painted rather than carved and is peeling very badly.
An augmented photo confirms enough details to establish that the headstone matches the District Council of Peterborough's cemetery record for section Protestant 3, plot # 479, that being of Harriett Susan Gay, died June 1925; her daughter, Olive May Irene Alchin (nee Gay), died December 1939; and a child, Jeffrey Alan Alchin, died 1949, who appears to be a son of Olive's widowed husband to a second marriage.
SA birth and death records confirm an obituary for Harriett that identifies her as having been the daughter of William J Garrard and Elizabeth Jemima Potter, and that Olive May Irene Alchin is her daughter to William Herbert Gay.