Just had an email from Northern...
Although I'm sure I've tried around 5 minutes before departure in the past myself a couple of times and it never actually worked!
From Monday 4 November, Advance Purchase tickets will be available to purchase up to 15 minutes prior to the train's departure.
Advance Purchase tickets are available on the Northern website, Northern app, at our ticket offices and from ticket vending machines in stations.
For some customers this may be a change as Advance Purchase tickets have previously been available up to 5 minutes before departure time on the app and website only. Please note Anytime tickets will remain available to purchase, up to immediately before travel.
Although I'm sure I've tried around 5 minutes before departure in the past myself a couple of times and it never actually worked!