Yes, it would have been helpful had there been information regarding this on the website.
Looks like I'm knackered for a ticket. Even though I got my Mansfield ticket online the system doesn't recognise that, so I can't get a Man City one online. I haven't got a few hours today or tomorrow to get on a train, queue and get a train back home, just got bigger priorities for the next 48 hours. If there's any left on Tuesday I'll just have to get what I can.