During the Test

You'll be given an eyesight check before you set off. Make sure you bring your glasses if you need them.
Your examiner may come across as "business-like", so don't be surprised or intimidated by this. He's done this hundreds of times and he's not your mate. He's there to examine you demonstrating Advanced Riding skills.
The examiner's a human being and knows you're nervous. He'll give you 5-10 minutes to settle down, so don't get freaked out if you make a few, early mistakes (so long as they're not bloopers).
If you make a mistake, don't obsess about it - and keep going! Don't let one mistake ruin an otherwise great ride. The examiner will cut you some slack and it may only be a discussion point after the test.
LAM Observers coach you to be above test standard before we recommend you sit your test. Trust in the SYSTEM and you'll do great!
Check out the IAM "What to expect on your test".