This post is based upon personal experience, supposition and guess work. Bits of it may not be based on fact, but here goes. Sources are a combination of me, the Chiltern Railways website and the National Rail website Live Departure Boards.
It does not make pretty reading, when you consider that the new weekday timetable kicks in in about seven hours. These delays were all caused by railway infrastructure issues (not train breakdowns).
It would appear that, just over an hour ago, something caused the majority of trains South of Banbury to grind to a halt. Or not leave their starting stations. A few trains seem to be moving now, but some are terminating short and there have been some cancellations. The reasons are either 'Emergency Engineering Works' or 'Signalling Problems'.
A lot of people need to cross their fingers very hard that whatever has gone wrong is sorted good and proper by the morning.