As seamusjbyrne says, the stadium gigs seem to get busier each year. I am going to Nottingham this year for the first time, but previously have always seen them at the NIA. My first year, 2006, I would guess about 60-70% of the seats were occupied (taking into account a sometimes fickle memory!), '07 probably about 70-80%. I didn't go in '08, but last year I would guess near enough a sell out. At each gig I have been to at the NIA, people were buying tickets on the night, so this may also happen at this gig.
Even if only half the seats are occupied, I think we will be in for a great night!