check the temperature sensors
make sure the operators are not operating them in a manner that will overheat them (such as pushing loads on the ground)
check the brake adjustments and make sure they arnt binding
make sure they are charging the machine properly (low volts = high amps and can damage circuits)
i see youre in the uk so im assuming they are 36 volt or higher
if they are 24 volts some ac inverters have overheating issues at this low of a voltage (we sell em cause americans like 24 volts mainly because of cost savings upfront.. we make alot of money from decisions made solely on upfront cost savings lol)
check for shorts and make sure all spike suppressors are working and installed (on horns fans relays etc)

"have a good luck"