The watch made the connection from the positive terminal of the uncovered fuse box to the throttle body. But it gets better - To get the spanner I was using onto the oil-filter fitting I was doing back up, I'd had to wiggle my arm down in the gap and jam it there. When the watch made the connection, it wasn't just a simple case of pulling my hand away. It was stuck for what seemed like ages.
Lesson well learned. No jewelery when working on the car and ALWAYS use the battery master shut-off.
More than a month later, it's still not great.