According to Hodges, Turing compared some modern custom to the torture and killing of the last to arrive at Celtic councils. I can find no other mention of this elsewhere; can anyone expand upon or at least substantiate this?
It's a well known fact (or legend), told by Julius Caesar in his Commentaries on the Gallic War (Original title, "De Bello Gallico") Chapter 5-56:
quo lege communi omnes puberes armati convenire coguntur; qui ex iis novissimus convenit, in conspectu multitudinis omnibus cruciatibus adfectus necatur
which can be translated as
Such is the custom of the Gauls in order to wage war: they oblige by law all young men to present themselves armed, and to which the last one arrives, at sight of the whole council, they dismember him.
It's not completely sure that this custom was real in Caesar's times. Maybe it was merely a rumour that Caesar took for granted. This comes under the legends that the Celts (who included the Gauls) liked to tell about themselves. See Táin Bó Cúailnge for an entertaining example.
Turing would have been familiar with Caesar's Commentaries on the Gallic War, because it was widely used in teaching Latin in Britain in the beginnings of 20th. Century. When he was 13 he was enrolled at Sherbone School, whose teachers placed large emphasis to the classics.