My initial reading was 43680 and they have estimated it as 44320 after one month. On what basis they have estimated?
They have estimated based on information on historic usage given by your previous supplier.
But, frankly, the issue you’re complaining about is of your own making. The idea is you submit meter readings monthly and then you always get an accurate bill. Why didn’t you submit a meter reading?
Also I have been continuously monitoring my Meter and it's numbers are increasing quite fast. for example during the whole night when everything was switched off apart from Fridge and Freezer, the meter reading went up by 6 units. Does these 2 appliances consume lot of energy?
Assuming “the whole night” is 8 hours then 6 units works out at 750W average. That does seem a little high. My house background usage runs at around 250-350W with everything ostensibly “switched off” except the fridge, freezer, boiler, and a computer server.
Is your fridge or freezer particularly old? They used to use awful amount of electricity. Modern devices are much more economical.
What else do you have in standby? Sky box? DVR? Games consoles? What about a hot water tank that might be heating on a thermostat over night?