After much pulling of hair, gnashing of teeth, searching on Google, etc…
I have finally got the /etc/aliases file under control :)
As some background, all mail historically went to the one mailbox, and was separated out by the client, and all was good with the world. Then I set up a proper mail server, with more than one mailbox, and everything got silly. Due to the huge range of local usernames we have used (mainly to track who is selling the data), the aliases file was getting beyond what I think of as reasonable.
It finally hit me that we were separating the mail using regexp, and the first 2 or 3 characters was enough to identify the intended destination. Armed with this (bloody obvious) insight, I started looking at getting exim to parse regexps in /etc/aliases. First port of call was to switch from lsearch to wildlsearch, which was great, until I noticed everything was falling through to the catchall mailbox! Rolled the changes back (that’s what notes are for, after all), and left it on a back-burner for a while.
Suddenly (at about 23:00!) I recalled seeing something about “real_local” (yes, I can be somewhat dense at times). Five minutes later, I have it working exactly as planned :)
Of course, that five minutes should have happened in the first place, but at least I got there!

For the record, the settings are:
data = ${lookup{$local_part}lsearch{/etc/aliases}}
data = ${lookup{$local_part}wildlsearch{/etc/aliases}}

^user.*: real-user