Managed to peek through one of the gaps in the bottom splash guard and spotted a broken connection to the gearbox earth.
The spade connector is still attached to the gearbox, but the cable has broken clean where it attached to the spade connector.
I'm guessing that this cable doesn't lead straight to the battery negative, as only one cable is attached to the battery and the car starts and runs fine.
The questions are:
1) Where does it terminate?
2) Is it worth just getting my local garage to solder another connector on to the same cable?
3) Should I just buy a new cable and do it myself? i.e. Is it an easy enough job?