dewdrop,
As far as I know, the skill test is divided into two parts. The upper air work, and the Navigation test. If you fail one, then you only have to repeat that one.
Further more, if the examiner is not happy with a particular manover, such as a steep turn or stall, he can ask you to do it again, there and then. The two exceptions to this I believe are the PFL which you must get first time, and Navigation...you must not get lost...you must always know where you are.
Don't worry about it too much. You will make some mistakes on your test...we all have. You will get questions wrong before the test....we all have.
The examiner will approach the test with two things in mind.
1. He will assume that you are a competent pilot, as otherwise your instructor would not have put you forward. Therefore little mistakes are forgiveable.
2. He will try to empart some of his knowledge to you, that your insturctors may have missed. They might do this by the questions that they ask in the begining, so don't worry if you don't know all the answers...they don't expect you to.
dp