I'd quite like to know the answer to this as well. I'm pretty sure the Bargain Berths are updated on Fridays, but it would be helpful to know the exact time!
It has come up on a thread before as to whether they are valid from Crewe / Preston, and someone posted that they weren't, after they emailed ScotRail.
However, I have used a FlexiPass from Preston on three separate occasions without problem, and this clearly says London Terminals on the ticket.