Invisibility cloaks were another attractive option too – it would save her having to duck out of sight when any of her numerous ex-boyfriends got on the same train carriage.
Sadly, when the Gods of Magic allocate powers they tend to do it indiscriminately. Ex-boyfriend number 94 (the stalker with a particular Tilly-borne grudge) had just got on the train. Undetectable to anyone, he sat behind Tilly nursing his grievance and his knife.