AT 230 VOLTS YOUR CURRENT DRAWN WOULD BE 16.4 AMPS. IT'S JUST POSSIBLE THERE'S A PROBLEM WITH THE ELEMENT IN THE HEATER, I.E. one part of the element is touching another part of it hence slightly reducing the resistance. It should be at around 17 ohms for operation on a 13 amp fuse (in a fused connection outlet). If this fault is present the rewireable fused will hold on to the overload as it is slight. Open the heater and check the condition and placement of the element, check with the manufacturer as to what the ohmic value of it should be.