No Hot Water - PulsaCoil 2000

Problem: My Pulsacoil 2000 doesnt give me hot water in the morning. I have to switch the unit off for a few minutes and then I have to switch it back on to get hot water. This has become a daily routine.

Additional information - In the morning, The unit is hot, the pipe is also hot. The overhead tank is sufficiently full. When I switch it off for a couple of minutes and switch it back on, I get hot water. My electricity supplier is Bulb energy and it provides me with single flat tarrif. I had the same problem couple of months back and I got a qualified engineer to look into the problem but he too couldnt detect the fault and had to make 3 visits. He then changed the sensor wire and then the problem suddenly went away. I have the same problem again - My Pulsacoil 2000 doesnt give me hot water in the morning. I have to switch the unit off for a few minutes and then I have to switch it back "on" to get hot water. This has become a daily routine. Can you please tell me what the problem is and what is the likely cause of it?