Okay... so Ryanair hide a lot of charges but this deal seems to beat the previous one of £10 one way. Yes - you have to pay extra if you don't pay by Electron, if you want hold luggage and/or if you don't check-in online but it's still a pretty good deal.
One of the £10 flights I was looking at yesterday has now gone down to £1 so it seems genuine enough.
In order to avoid the card charges I bought an Electron Travel Card from the Post Office for a charge of £3 (1.5% of the amount you put on the card or £3 whichever is the higher up to a max of £20) so hopefully that will remove the £4 each way charge for using other cards.... I will find out today after I activate the card. If you have an Electron card from a bank then even better.
EDIT : The TravelCard option works (see post #5)
The general conditions seem to be :
Booking Period: Tue 28th Oct 08 - Thu 30th Oct 08
Travel Period: Tue 11th Nov 08 - Sat 31st Jan 09
Applicable Days: after 10am Mon / all day Tue Wed Thurs Sat
Subject To Availability
Flight must be purchased: 14 Days (subject to availability) in advance
Blackout Period: 18th Dec '08 - 07th Jan '09
Notes on this fare: All Holidays, School Breaks and Major Sporting Events are excluded from this offer