Go with Sebastian, it's a lovely name (although I am biased as I have a Sebastian Alexander)known to his friends as Seb and his little sister as Sebby. He's definately not a toff, more of a "dude" amongst his friends. I was inspired by Seb Coe and Sebastian Conran. No regrets on the name choice (other than someone I know well CHOSE the same name 2 months later and they now attend the same pre-school 
. The only problem I find is that some adults have massive problems, no idea why, in saying Seb and he quite often gets called Zeb, even when they try to pronounce Seb they can't!!