Sure enough, he was right on the money! I had wrongly assumed that the ground connections were all on a common strip, but in the end they were isolated terminals. We had to fashion a pair of jumpers to spread the load across multiple terminals, and it worked!
We finally dragged the plane out of the hanger and it fired right up! (well once I switched on the mags that is)
Last thing for the night was a complete washing top to bottom, nose to tail.