Hey Jonny,
I just passed Stage 1 too, the email I got says that you have to book a date within a week, but the date you book does not have to be within a week (i.e. if you got the email on April 25th, you have until April 2nd to book a date, but you could choose an interview date of May 15th)
(Having said this they all state in the email that due to limited date availability at the moment the 7 day rule may not apply)
... I just followed the link and the next available dates are in June anyway :/