No water from the Economy 7/heat wise immersion.

Hi. I've recently been away from home for 10 months so the system has been turned off. I replaced the header tank recently because it was leaking and turned everything on expecting it to heat up.

I've had a look and the cheap eceltricity immersion isn't heating up the water. I turned on the boost immersion and the system started to heat but this is a very expensive way of heating water.

I've had this happen before but it was such a long time ago that I cannot remember what the cause was but I have a flickering memory it had something to do with the circuit board but I wouldn't bet my life on it.

Has anybody got any suggestions what could be the cause of this? In selling my flat and don't want to leave the next owner a Brocken boiler.

Hi just to add the header tank is filled and both LED's are on.