Info courtesy of the 'Money Saving Expert' mailshot:
http://www.moneysavingexpert.com/dea...als#Southern90

---quote---
On Thurs 26 May only, get 90% off Advance single tickets from Southern
Railway. You can buy tickets for up to 12 weeks ahead (14 August 2011). Plus
Senior, 16-25, Friends & Family, Disabled or HM Forces Railcards can be
used!
[...]
Southern has said around 750,000 tickets are available, but they're still
likely to go fast. Be warned, the site may be slow from heavy traffic.
---/quote---

Must be purchased from the Southern website:
http://www.southernrailway.com

They did a similar promotion a couple of years ago.

Note that it's only for Advance tickets on the Southern network, which
aren't available for all journeys/routes.

Also worth noting that you may get better results for journeys to/from
London if you specify "London Victoria" as the destination/origin (rather
than the generic "London").

Plus it's worth bearing in mind the Southern Daysave and Southern Downlander
tickets:
http://www.southernrailway.com/tickets-and-fares/daysave/
http://www.southernrailway.com/tickets-and-fares/downlander/