Lucky you that you dont need heat at this time of year!! Storage heaters work by heating bricks in the heater at night, on a cheap off peak rate, then dissipate the heat out during the day..
To switch them off, just turn of the switch on the wall beside the heater. Simple.
The time clock, i,m nearly sure, can be switched off by switching off an mcb (circuit breaker) in the mcb board. This will be marked, but if ur not sure, just wait till 11 o'clock and switch it off then, you'll hear the buzzing stop. It generally is a 6amp mcb, but i have seen 10amps used.