Why does my Mira Sport 10.8Kw shower keep tripping on High?

I have a Mira Sport shower rated at 10.8Kw which has been installed and working perfectly for 3 years. it has now developed a fault where it trips the current leak detector when set to high. The trip occurs approx. 20 secs. after switch on. If the temperature is reduced it still trips but after approx 8-10 minutes. Mira have changed heating element and pcb and advised I change 45 amp pull cord. have now changed pull cord fault still apparent. Any ideas??