I always ask when I book. I very much doubt that the receptionist bothers to tell the driver anything about it, so I make sure I have the right change to hand over and if the driver asks where I got the figure from, I tell him. It's not as if he can do anything about it if he disagrees.
I could be wrong here, but I doubt there are any 'right' fares with minicabs as there are with metered taxis; it's down to the company to set what fare they want to Gatwick (or wherever), and you're free to shop around other companies if you think they're asking too much.