Water meter vs unmetered

KxMx

I'm a single person living in a 1 bed flat, no bath or dishwasher. Average 4 showers a week, 1 load of washing weekly per summer, 2 in winter. 

My water company estimates my use at 40m3 a year,
The calculator linked to estimates 38.

I've been here 7 months now and my usage is steady at 2m3 per month including summer (more showers) and winter (double the laundry loads). 
So my actual use is 24m3 per year. 


My meter is easily accessible so I submit a monthly reading.

With a 25% discount I am paying £12 per month.
My company bills annually but I self generate a bill every 6 months to make sure the payment amount is correct.