Thanks Richard, I'll start with the VDO Master Connector.
I spent some time poring over the Harness Diagram and figured out the following are earthed with the VDO Gauge. There's nothing else that I'd have noticed wasn't working so will also check the earth. Either way, unless it fails completely, it's just trying things.
Listed in case it helps anyone else.
B 230 - Diagnostics
B 231 - VDO Master Gauge
B 232 - Power Socket
B 234 - Heated Screen
B 235 - Rear Harness / No Plate Light?.
B 236 - Power Steering
B 237 - Heated Seats
