In reply to trueuk1: Following on from the good advise above...
Ignore the assessor/treat them as part of your group during the session you lead.
If you are ready for assessment (which it sounds like you are) treat the day as an opportunity to learn new skills and share you knowledge with the other candidates.
If you know where the assessment will be it may be worth visiting the wall, work out what are good lines to put people on, find out if there is a cafe/where the toilets are/how long it will take you to drive there. This will reduce any stress you may have and make you feel more relaxed and able to perform on the day.
Let us know how you get on! Good Luck!