We have recently booked and got a 10% discount early bird - we are going the week after Easter 09, and they had about 3 weeks that they were doing the offer for.
It shows v clearly on their website when you go to look at the dates - the discounted dates are highlighted in a different colour!
As an aside DH boss went last year - and re-booked another day while they were still there he loved it so much.
Its gonna be a real highlight of our trip - I hope!