Thanks, Bruce, I did check all of those things. Anyway, the problem is solved! It must have been the oil. Because it was brand new and the oil was so clear I had a hard job seeing when it was full and actually over-filled it to blazes. Also, some oil did spill around the filler and ran down onto the base. I can only assume that it somehow caused the short because after leaving the machine for a couple of hours I started it up and the problem had gone, and hasn't returned even after mowing about 20 square metres. One to chalk up to experience. Thanks for your quick response.
Cheers,
Chriso