My driving instructor friend tells me that most examiners are looking for reasons to pass rather than reasons to fail and that most are human and friendly.
Don't go to pieces if you make a mistake - you've got 15 minors to go at, so forget it and move on - just try not to make the same mistake.
Same if you don't follow the directions, the examiner will tell you where to turn to get back on the route, (or the satnav will if it's in that part of the test).
Don't be afraid to talk to the examiner if you think there's a problem.
When my son did his (after being taught by my instructor friend), he couldn't pull out after parking as there was a steady stream of traffic. Luckily the instructor had told him not to pull out if it wasn't clear, so he just sat there, as the minutes ticked by, wondering what to do. Apparently after 5 minutes or so, the examiner told him to indicate he was pulling out and to hope someone would let him in - he did exactly that. In the post test briefing, examiner told him he'd done exactly the right thing.
So, it's deep breath time, try to stay calm and remember what you've been told. It's quite short, timewise, so will soon be over!