Just realised that what happened with your connection was supposed to happen with mine... the difference is that Plusnet ceased my line too early in order for Zen to take it.
It was supposed to be that Zen took the line while Plusnet still provided my service which seems to be what happen with you, in this case it would mean that I would go offline, put in my user details (like I did last night) and then I would of been connected...
From what I've seen this morning, it looks like Zen took over the line at around 9am. so between 00:15 and 9am my line was dead!