So would I be right in saying that even if I fail on one specific point in either the navigation or handling section I'd have to repeat that entire section, even if the rest of the exercises are done well?
Are you planning to fail certain sections deliberately? Be positive. You have to make a very serious error to fail a skills test. If you have it in your mind that you will fail then there will be just one outcome. If the unlikely happens then worry about what you have to do on the next attempt, but not before.
I was very nervous when I did my test earlier in the year, but the last thing on my mind was thinking about failure. I was too concerned with trying to memorise all sorts of things that I would need to remember on the day of the test. I made a couple of silly, minor mistakes on the test but I knew immediately and told the examiner. He was happy that I had identified the mistakes and because of this it didn't affect the outcome. The most important thing, that others have said, is don't dwell on any mistakes for the remainder of the test. Fly each section of the test as though they are mini individual tests.