Don't rely on the internet access at the sales desk.....very restricted. Also there is a cut-off point at the airport so "very last minute" purchases are not a possibility.....allow a couple of hours. And as per the previous post, late purchases tend to be the most expensive, so book as early as you can.
The call centre option is very good if you don't want to book over the internet. (Just don't phone in the middle of cr@p weather or disruptions as you'll be hanging on......!)