Hmm, I'd rather not try reflashing just yet, because I think the guy I bought it off did that before sending it off to me.

I've done a few more tests, and I've found a consistent pattern.

If I plug the phone into the PC and turn it on whilst it's connected (from being completely off), both charging and mass storage work correctly, even if I unplug the phone for a while and connect it again later (just so long as the phone isn't shut down).

When the phone is turned on when it's NOT connected to the PC, it won't connect to the PC AT ALL (it also won't charge), and there's nothing I can do but restart the phone with it connected to the PC.

So it sounds to me that some driver on the n900 isn't loading unless it's connected to the PC when it starts up. Unless it IS down to a hardware fault... this is why i've come to this forum for some help