In reply to Ali:
I picked up a pair of Trek Circuit shorts in the sale at JE James and they're fantastic. They're about £40 full price but I got them half price!
The pad is kind of 'articulated' and is really really comfy. (looks a bit like the fancy red sugoi (?) ones on Wiggle.)
I've done a couple of 50+ mile rides in them and whilst my legs were shot, my bum stayed pretty much comfy. I'd definitely recommend them.