Good point made by Prowler and Kertez.
With Serco you have to "earn" your off duty days by working the preceeding days as opposed to having them "rostered" as a part of your shift cycle (depending on where your from)
E.G. If you work 2 afternoon shifts then you "earn" the next day off.
If you take those 2 afternoon shifts off as leave, then you also have to take the third day as leave becaouse you havnt earned it yet. That is how the 56 days get chewed-up.
Just something to bear in mind when you are considering the package, you still get plenty of leave to use, as long as there are enough staff on station to allow you actually take the leave, and it can sometimes be a lottery with getting dates that actually suit you.