As Dave suggests, a long travel hardtail will work well (I did it in 2009 on a Blue Pig) but you do take a pounding over the duration of the 7 days. A full sus that you can carry would be a much better all rounder for the event. It also depends a little if you’re doing it for the experience or to compete at the sharp end – something more bouncy will be better for many of the timed stages, which are predominantly down, but not always.
You will however be going uphill a lot of the time – the word ‘updulating’ was coined to re-define Ash’s use of rolling or undulating as a description of the days riding.
Practice walking/climbing with your bike before you go, and build up some leg muscles for walking too. Take comfortable shoes, and if using SPD’s make sure you’ve got flexible, grippy soles and not race shoes.
Swinley is not a great place to train for the event no matter how much you may try to convince yourself. It didn’t work for a number of us who went in 2009!