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Ah hah!

Turns out, there is an option in mythtv-setup, under "General", labelled "XMLTV offset". I changed this from "none" to "auto", cleared all the data, rebooted and ran mythfilldatabase...

I'm using this script:
http://www.cse.unsw.edu.au/~willu/xmltv/tv_grab_au_reg

as per these instructions:
http://www.cse.unsw.edu.au/~willu/xmltv/tv_grab_au_reg.html

to grab tv guide data for...