Hi Pipergirl,
I did my FI test about 18 months ago, and as other posters have commented, learnt a great deal from it.
I look back on it with enjoyment, but actually on the day was quite nervous. Relaxing is the key, but is very difficult under test conditions. I nearly blew a Sim test for an airline the other day due to being too tense - but thats another story! I find that actually repeating relax..relax..relax in my head just before, or even during a test works for me.
I remember on my test I was given ex19 - intro to instrument flying- as the unbriefed exercise. It was the single exercise I had practiced the least in my FI course, but muddled through and still passed. The examiners are huran and do not expect perfection....so relax, and best of British !