I don't know if this is a general problem or simply a quirk of the fact that my bill is produced in one month whilst my direct debit payment is called at the start of the following month, BUT …..
After monitoring it for a couple of months, I have noticed is that the 'minimum monthly payment' in the Direct Debit Calculator changes quite dramatically (in my case by more than 10%) depending on when in the monthly cycle I use the tool – lowest immediately after my monthly direct debit payment hits the account and highest just before the direct debit is due to hit the account.
I am guessing it is down to how the tool works on calendar months, rather than billing or payment cycles, but surely the 'minimum monthly payment' should - barring tariff changes or dramatic changes in consumption - remain relatively stable throughout the month.
In the case of my account, my daily consumption charges are reliably with in £0.02 range at this time of year, so the absolute maximum range of fluctuation for the 'minimum monthly payment' should be less than +/- £1 .. not more than 10%.
BTW, I have also noticed that the thirteenth column in the 'balance' display of the online calculator, when accessing through a browser, is mislabelled 'balance at the end of the month' when in fact it is the 'balance 12 months from today's date'.
Given that the predicted balance in the thirteenth column changes in exact synchronisation with the changes in the 'minimum monthly payment' , I am hazarding a guess that the two issues are related and that the Direct Debit Calculator is using the thirteenth column's data as per how the column is labelled, rather than what the column actually represents.
It tends to suggest that depending on when in the monthly cycle you use the Direct Debit Calculator, it will calculate the minimum payment on anywhere between 12 months of usage and payments up to 12 months + another 30 days of usage (but still only 12 months of payments), rather than always just 12 months of payments and consumption.
Would it be possible to get the Direct Debit Calculator fixed?