Buy a power consumption meter and check each appliance over 24 hrs. I'd
suspect the freezers if they are old, but there's no substitute for a
lot of tedious measurements. The storage heater consumption seems a bit
low, is the clock correct in the meter? I found a large number(15) of
small appliances which consumed £2 of electricity/month, these really
can add up. Lighting can also be expensive if it is normal bulbs.

Regards
Capitol