not 'obliged' to give account info

i paid a lump sum on my account, then saw that even though my payment was deducted, my installment period wasn't amended on the next statement. i emailed and phoned the company a few times. got a reply today by email - they are not 'obliged' to calculate my installment period because I 'overpaid' my account. i phoned the call centre and was fed a load of bllsht about billing on the 1st of the month and interest and blah blah blah. took me with my limited knowledge 5 mins to calculate the remaining amount of installments, and my accountant did it in two minutes. what I don't understand and agree with is the phrase 'not obligated' to give me info about my account. these guys are ***********. stay away!!