if you're on d/d with EDF they always like to over estimate, then when they do the meter reading at year end, you get the final bill with a refund or demand for the balance owed..
your water heater may have a fault if it keeps tripping the cutout.. my old water heater was fine until i wanted to raise the water temp in the tank, in a attempt to give more available hot water for longer, i turned the stat a little.. and didn't think more of it until next day.. cold water!!.. the timer activated as per EDF signal, and the cut out tripped.. i reset the stat to 60 and all was fine.. i tried again some time later with the same result.. trip out.. new stats are reasonably priced, unlike the elements.. almost cheaper to install a new tank!! do you have the timer gizmo that is triggered by EDF in off peak time?.. make sure it's not set to manual/constant heat.. i'm no expert, but can't see how the external cabling can make your stat trip out.. you have power from the meter or you don't.. the stat is self governing!
if you're not on "Tempo" tariff, you seem to be using a lot of power.. if you have a rotary meter, count the revs and do the calcs
if you have the digital meter, there is a facility to see instant power usage going thru the meter.. i think it's sequence 7 on the buttons, whereby you reset the instant reading to zero and then you can count the units per time period and do the maths.. ideally, turn off all comsumer items you can, then do the test..
not really relevent, as every home is different.. i have a 25yr old 2 bed bungalow, log burner, normal electric needs and my bill is 750 tops.. as you say, maybe a good idea to check now.. Courage!