Tudor Athorn is the late uncle of Samhain Athorn, the man with Fragile X syndrome who lives on Albermarle Way in Clerkenwell.1

When Strike interviews Samhain, he tells Strike that his uncle had once confided in him that the local gangster, Niccolo Ricci, had killed Margot.2

“‘What did your Uncle Tudor say happened to the lady doctor?’

‘I can’t tell you that,’ said Samhain, suddenly grinning. ‘Uncle Tudor said not to tell. Never.’ But he grinned with a Puckish delight at having a secret.”3

^{1-3: Troubled Blood, Chapter 38}