You can't book in advance.
DD is very lucky to have had this experience twice, and both times we emailed we were told we had to book once we arrived. We are returning in two weeks and, just incase, we emailed the address above last week and asked if it was possible to book in advance now - and we got the same reply as always, that it has to be done in person and no guarantee of specific dates can be made.
We have never had a problem getting a slot on the day we wanted, booking less than 24 hours in advance.
HTH