Hi Rowan

Not sure how to put it into the exact words (I'm suffering severe brain fog today!) but I would tell your boss that you act as a kind of mentor for the other admins in the company.  I did the same at a job I held a good few years ago and made sure that my boss was aware of how much time I spent either tutoring other secretaries or just plain old giving advice.  "Mentor" was the word / expression that suited both activities and, though it wasn't an official duty & I wasn't the head secretary or anything remotely similar, people came to me for advice...  It's a good feeling!

I do hope this has helped in some small way.  Good luck with your evaluation and do let us know how you get on.