oil filled radiator wiring fault?

Hi,
I recently bought two new identical oil filled radiators off ebay. Both of them dont seem to work correctly. They have three heat settings 850w, 1750w and 2600w ( 850w and 1750w combined).
On the 850w setting the unit works fine, it heats up and responds to the dial thermostat and will keep a small room at temperature. On the 1750w setting the unit will heat up and the surface temperature of the heater will get hot ( I hav'nt measured the exact temperature) it will then cut out and cool down even before the dial thermostat has been satisfied. After the heater has cooled down it will heat up again and the cycle will continue. Therefore the room it is in will never get to temperature as there is a considerable length in time inbetween the on and off period. On the 2600w setting both elements heat up ( I have tested this just using a basic plugin energy meter) , the suface temperature of the heater will get hot then the 1750w element will cut out. The 850w element remains on. The 1750w element will not come back on as the 850w element is keeping the oil hot. Therefore the unit is running constantly at 850w ,which is not enough to heat up a larger room.
I have had a look inside the unit and it is wired as follows . I have not included the neutral and earth connections in this.

850w circuit is as follows Mains in > tilt switch > dial thermostat > rocker switch > 850w element.

1750w circuit is as follows Mains i > tilt switch > dial thermostat > rocker switch > overheat thermostat attached to body of heater > 1750w element.

Is this wired correctly? i would of thought that both elements would be controled by the overheat stat ?

If it is wired correctly, is the overheat stat vastly over sensitive? What temps should it be cutting in and out out?
The thermal cutout on both thses units are a KSD301 with a temp of 60 C. Surely this value is too low?? I have looked at the wiring and it would be very easy to swap the wires over so the supply to the dial thermostat has passed through the cutout thermostat, therefore both elements are protected.

I have since got a partial refund from the vendor, but I would like to get these heaters working correctly , any ideas??

Thanks in advance Peter