Purex Islander tripping out!

[Crewey]

Hi Guys, I have a Purex Islander Electric pool heater. I have just opened the pool up for the Summer and the heater keeps tripping out. I have checked inside and there are no loose wires. I turn the heater on and the botton two orange lights come on. A few minutes later the "heat" light comes on and the heater trips out! I have tried disconnecting the two wires from the top of the coil, but it still trips out.. I turned down the heat dial to the lowest setting and the two lights were on, then as I turned it up, the "heat" light comes on and off goes the electric. I am a little stumped and wondered whether anybody had any ideas. Any help would be much appreciated.
Thanks
Dave