Carver 2 Caravan Water Heater?

I have just brought a second hand 1995 Lunar Mercury in excellent condition. At the weekend I started testing the system out as we are off on hols soon. The water heater fired up first time no probs - the green light was on & the water started getting hot. After about 30 mins I tried the tap & the water was scorching hot so I ran it for a bit & the heater tripped out & the red light came on & despite following all the trouble shooting instructions it refused to start again. I gave up.
The next day I tried again & exactly the same thing happened again.
Today I tried again & guess what - exactly the same. So I thought it maybe it was overheating (due to the very hot water) and tripping out.
I took the cowling off and was going to adjust the temperature as per the instructions but the flap I was supposed to lift to find the adjustment knob is not there! So now what?
Anyone got any suggestions - apart from switching it off early to avoid over heating?

Ali