Depends what year? A long time ago I was told it was 10pm but more recently I think it has been nearer to half ten. The only thing is I'm not sure how flexible the curfew is or if it is set in stone..?
I remember when they used to put meetings on during world cup week and there was an individual at Poole with Championship standard riders, it didn't finish til about 11 or half past...? This would have been a long time ago as I think Chris Neath was in the line up! Prob why there is a curfew now!