If i had my book i could tell you :? The length of the cable would also come into it but i would imagine it would take a fair bit of current before it caught fire.
.75mm flexible 240 volt cable is rated @ 7.5 amps @ 240 volts, that = 1800 watts, if switching through your thermostats which are rated @ 5 amps (resistive load) then the maximum load would be 1200 watts. Good idea to allow a safe margin for error and not exceed say 900 watts.