Thanks for that Piers, but no joy... the USB drivers in the Yarvik LiveSuit weren't recognised, and Windows insisted I had the best drivers (forgot to mention I'd downloaded the 64 bit version of PdaNet).
Anyway, after extensive faffing about, I tried a different USB port (yeah, I know, that should've been step 1 in my troubleshooting) and it all burst into life. I then tried a different USB cable, and that worked on the original port as well as the "new" one.
So, the moral seems to be, despite being able to copy files back and forth happily when the device is mounted in "file copy mode", it's very fussy about the USB connection when using adb.
Cheers,
Pete