Reduce transaction log file utilisation
It looks like replication was not set up properly; I'm guessing that that means that when a transaction gets written to the log and replication fails, the log entry doesn't get marked as ready-to-be-cleaned-up, so the entries just pile up and the log utilisation goes up with it.
I deleted all replication configuration, and the log file utilisation dropped from 99% to 0.1%, so now I can drop the log file back to something more manageable.