Fixed!
So I thought I'd take a break from revising and tear down this printer to find the problem. It turns out that the gear wheel connecting the toner drum to the main driver motor had slipped, meaning that the drum wasn't pulling the paper through - causing the jam! I've attached some tear down pictures of the mechanism.
I simply put the mechanism back together and the gear turned again. 10 minutes later and reassembled, it's working fine and printing duplex without a problem. In fact, I've now switched to this printer because it doesn't have annoying sticky buttons like the other 2330dn. Thanks very much to stratman for digging up those firmware files for me. I've created a list of mirrors for 2330d/n firmware for future reference.
So I now have two fully working 2330dn laser printers. Surely I can't need both, right...?
Cracking good job! Great job on publishing a web page with the firmware versions and tool. Like the pics, too. Your post may help someone else one day. I am impressed.