Ah, you did mention that (I've been helping a mate move house today, so am currently a bit tired!).

I'd be more inclined to try it with an ammeter, as that way things are still tied to earth, and give a better idea if it's something slowly building to tripping point, or something causing a more sudden spike. Of course an oscilloscope would be better, but you may as well try with what tools you've got to hand first.