Also, get Met out of the way ASAP.
Ideally before you sit the Nav exam. Not only will you need it for your actual nav trips but you can also self-brief the day before your lessons. This will mean fewer phonecalls to your instructor on the morning of your lesson along the lines of 'are we going to be flying today?' Instructors like to see a bit of initiative.
With the Nav exam itself, you will generally need to pass it before you start the solo nav part of your training. But the theoretical side of the nav exam is much easier to pass once you have some practical experience of navigating. You might also find that your instructor's methods of teaching nav might be much more straight forward than those methods found in the textbooks. Mine certainly were!
