Thanks. I can't see anything to turn off the auto sync but I have unchecked the preference to automatically link my device to my library. I'll see how that goes.
It's such a shame as the device itself is a delight to use (easily the best form factor I've come across in a gadget in the past 10 years) but at this point I would not recommend it to anyone based on having to use that software. I know it's possible to use something like Calibre but that doesn't seem terribly feasible given how few DRM-free books are available (at least here in Oz) and I don't fancy having multiple bits of software for the same function.
Sigh, life is never perfect.