Hi Simon
You have your on-site folder ??? Do you know what you have to provide in the way of evidence ?? So what are you going to show them ? For example what PPE have you got ready can they see you drilling a hole with it all on ??
You need to be pro-active !!!
As Chris says, what's the job they are coming to witness? Make sure you have full PPE at all times and think it through so it goes straight forward, last thing you want to do is make a stupid mistake in front of an assessor.
When thinking PPE, make sure you have steel toes, knee pads, safety glasses, ear defenders if needed etc.