The heater and water heater will use most electricity. If you look on the back of most appliances, it will say how much electricity it uses. A small heater has a 1kW power so, if it's on for an hour (without a thermostat), that's 1KWh or one unit (or 10p or so). The water heater will probably be 3KW but with a thermostat, the usage per day is harder to work out. To check your meter, turn everything off except something that you know the power of and leave that on for a period of time. If a 1KW heater adds one unit to the meter in an hour, your meter is fine.
It's worth checking what the meter starts and finishes at for their calculation. It's not unusual for suppliers to use silly estimated readings.