Jane Asher portrayed Sonia in 'Time for Murder's 4th episode of the first season, titled 'Bright Smiler'.
Avon Eve (Janet Suzman), a successful writer for films and television, checks into a health spa for a week's rest and recuperation. There, she is assigned a masseuse, Sonja (Jane Asher), who the director of the spa calls the Smiler for her constantly pleasant and smiling disposition. Sonja has heard of Avon and is particularly enchanted by one of her works for television - one that Avon thinks is quite dreadful.
As they spend more time together, Sonja begins to remember a failed love affair with Sir Jonathan Sibley (Jeremy Nicholas) who claimed to have loved her but refused to marry. When her old love arrives at the spa with his wife - and fails to even recognize Sonja - she becomes unhinged and plays a terrifying game of Russian roulette.
Photo 1) Daedalus books.
Photo 2) From the net. Comment below if it's yours, and I'll give you full credit.
Photo 3) IMFDB
Photo 4) Confabulous blog.