Sarah Shadlock makes her first appearance in The Silkworm, at Matthew’s mother’s funeral. She is a friend of Matthew’s from their university days, and she is first seen embracing Matthew in a way that Robin thinks is “a little more lascivious, perhaps, than was appropriate in the circumstances.”1
In Career of Evil, after socializing with Sarah and her boyfriend, Tom, it is observed that Robin and Matthew always seem to quarrel. Robin regards Sarah as a “shit stirrer,” who at university “had tried as hard as she could to win Matthew away” from her.2
Physically, Sarah is described as looking like Strike’s girlfriend Elin, with Nordic-blonde beauty (but Sarah’s not as beautiful as Elin, in Robin’s estimation).3
In Lethal White, we learn that Sarah is recently engaged to Tom Turvey.4 Still disliked by Robin, Sarah continues in her flirtatious attitude with Matthew and general “shit stirring” that Robin has previously referred to. Sarah and Matthew often team up to tease Tom about his baldness, which Tom is very subconscious of.5
We also learn that Sarah works for the auction house, Christie’s.6 A fact that later comes in handy for Robin to gain information on art during the Chiswell investigation.7
We don’t have the pleasure of seeing Sarah Shadlock in Troubled Blood, but Robin learns shortly before Christmas that her engagement to Tom Turvey is over and that Sarah and Matthew are now an official couple.8 Robin’s mother, Linda, is furious when she finds out that Matthew and Sarah are walking hand in hand through Masham while at home for Christmas.9
We also learn that Sarah’s signature perfume is Flowerbomb. Robin thinks of the scent as “loud and sweet” and, interestingly, the bottle is shaped like a hand grenade.10
In The Ink Black Heart, Robin is told by her cousin Katie that Sarah and Matthew spent Christmas in Masham with their new son, William. “I really don’t like that Sarah,” says Katie. “Sooooo smug.” “Can’t say I’m that keen on her either,” replies Robin.11 We agree with you, Katie and Robin!
1: The Silkworm, Chapter 32
2: Career of Evil, Chapter 2
3: Career of Evil, Chapter 4
4-6: Lethal White, Chapter 7
7: Lethal White, Chapter 68
8: Troubled Blood, Chapter 24
9: Troubled Blood, Chapter 28
10: Troubled Blood, Chapter 12
11. The Ink Black Heart, Chapter 4