Just a quick follow up:
I've spent the best part of the day playing with Duplicity version 0.6.06 and haven't had any luck with getting the resume feature to work properly (either with the perl script / web admin or directly from the CLI). The good news is all the basic functionality still works but that's hardly and incentive to upgradethe package
I've posted a couple of questions on their Launchpad page but in the meantime I've rolled it back to the version in the repositories.
Should you ever decide to increase the functionality of the backup script (please do!) then I'd be happy to to do some testing for you.
In the meantime here's my kludgey workaround for getting big backup sets backed up to my online storage (I'm trying to backup 100s GB and can't practically do it in smaller backup sets. Obviously this will take days and my ftp connection keeps getting lost after a day or two for different reasons - hence the request to get the new version of Duplicity working). Fortunately, my online storage provider has not just ftp but also rsync access so for the minute I'm running a backup to the Bubba's local ftp server (I wasn't sure if that would work) and I've wrote a script that rsyncs the directory I'm saving backups into to the online storage. The script relaunches every hour if it drops out for whatever reason. Once everything's online I can then remove the script and the local backups.