Decrease in interest rate takes effect a month later with no refund

The interest rate decreased by .25 % in November yet on the 1 December, my instalment did not. Marquis Finance emailed me stating that they did not have enough time to decrease the instalment and my decreased instalment will take effect on 1 January which did happen. However, I did not receive a refund for the extra amount paid on 1 November. I emailed the company...no response to date...very bad service with an even worse excuse for not adjusting the new premium on 1 November...yet I'm sure if I had paid them less than expected, I would have received many calls ....