It's been a while since I last did this, so things may have changed.
I don't recall getting a discount, but the best aspect of the group booking was that you could put the names of passengers in just a week before departure, which means that if someone drops out, you can sub in another passenger.
Useful to know Thirsty. Sub in another passenger at no charge on a large group where things could pop up is good. Do you know if this applies to just one person dropping out or a few if the group size is say 25 people.