Having seen the kit list provided by Andy, I would say that if he wants another lens, then a wider angle is probably the way to go. Of the options available, I would say that the Sigma 10-20 is probably the one to aim for, although if the OP has any aspirations towards full frame in the future the 12-24 Siggy is possibly the only choice.
ETA.
The Sigma lens listed is only f/2.8 at the wider end, dropping to f/4.5 at the long end. It's also only 1mm wider than the kit lens (although the extra 15mm at the long end fills the gap he has).