I've run the car for 4 meeting now and I've still got the kit ones in.
I've been told AS6581 ASSOCIATED DIFF BALLS 3/32 CARBIDE are the ones to use at only £12 for 12 from loads of places.
I've also seen some ceramic ones for 99p each somewhere with free delivery.
The Kyosho ones are about £17
Still not sure which ones to get though....
Brett