Truma Ultraheat only works on 500 watts

Hi all, 

my wife and I went to Chirk in ours caravan over Christmas and our Truma Ultraheat decided to play up. We had the gas heater to fall back on, so it wasn't a huge problem. It works on the 500 watt setting but not on the 1000 and 2000 watt settings. Has anybody else had this problem, and if so what was the Solution. My instincts tell me it's something to do with the electronic control unit.

Regards and thanks

Howard