Got out of bed this morning to check the phone, and it's back up and working. No idea if a Telstra tech was still coming out, or if he's done something, or what... but it's working.
Rang up Telstra to cancel the ticket, and they weren't terribly forthcoming about what was done. I don't really care - after cleaning up some of the wiring last night and moving the ADSL modem and the FreeBSD router out to the room where the phone line terminates, running a speed test gives us about the best result we've had in a long time.
Hopefully it stays this way!