What MJ says is absolutely right, Ian. I had a bit of a 'mare on the day of my test for a number of reasons. I had got myself into a real tizzy, convinced I wasn't ready.
I was. I passed. I even cocked a few things up, but fixed them immediately. The examiner really isn't looking to catch you out, if you make a mistake then say so and fix it. Unless you've
really scared them, you'll be fine!
Good luck!