Thanks for your help. I've checked all the wiring, everything looks pristine (this was the first time it had ever been opened). All the plugs and sockets in the path from the keybed to the first board marked DM and from there to the left hand front panel card marked PNL look fine, no bent pins or loose wires. I'm guessing that I'm going to have to get the multimeter out and start checking continuity in the wiring harness. I was hoping that this was a stock fault that had been seen before.