The drop-down list of names that Outlook gives us to auto-complete a name or address being written is stored in a file called
(They're called Ingressor, if anyone needs those tools)
It turns out, however, that the most common usage for such an editor isn't necessary. If we just want to remove an item from the cache we can do it from within Outlook itself – simply navigate the drop-down options using the arrow keys and press Del on the one we want to erase – and away it goes.
Very simple, and even logical when you think about it. We're just not used to live editing of an auto-complete list.