"and 'Gertie' Lawrence and Augustus John and Carpentier"
Augustus John (1909)
Public DomainAugustus John (1909) - Credit: Jacob Hilsdorf

Rex Mottram moves in diverse circles:

Gertrude Lawrence (1898-1952) was a British actress and musical comedy performer;

Augustus John (1878-1961) was a Welsh artist, particularly well-known for his individualistic style of portrait painting;

Carpentier is probably the French boxer, Georges Carpentier (1894-1975).