Good to know Im in the clear about the CO2 tanks. No need to apologise, Ed - if you have concerns you have to raise them even if you dont know everything about the field in question (who does?)

Whatever remote kill system you put in place can of course fail but if you make your weapon or drive reliant on more than one system (speed controller plus relay for example) and you keep them as separate as possible you drastically reduce the risk of a runaway robot.

I will see if Maplin have any of the 180A relays tomorrow for my robots. I suspect that space rather than weight will be a problem for many existing robots though.