I've been paying my store card balance off at £150 per month for six months and each month the interest charge has been approximately four pounds less than the previous month. However my latest payment was £250 but the interest charge on my latest statement seems only to have reduced by four pounds, the same as for a £150 payment. I haven't made any purchases. Are they right?
I think that you might find that the next month's statement shows a greater reduction because the interest would have been calculated "in arrears" ie on the previous balance, before your £250 payment.