Here's a pic of the broken brush wire, which I suspect was shoving volts into the outside of the can and messing up my servo (via conduction through the graphite chassis). Bear in mind that the brush wire used to be connected up, albeit with only a few strands of wire connecting it together which I didn't actually notice until after I'd run the race and experienced the problems.
And here's the pic of the comm (all a bit burned after about 6 laps)....doesn't look good does it!?! The comm was freshly skimmed and golden (honest!) before racing...now it appears to be a bit burned.
All other tests with other motors so far suggest that the motor is at fault.
Cheers.