Rogues

We rented a property from Just Letting Westville. We paid a rental and a utility deposit prior to moving into the property. We moved out of the property end if November 2016. The utility bill runs from the period 15th if the month to 15th on the following month. The whole utility amount was deducted from my utility deposit ie for the 15 November to 15 December 2015. I was not sent the bill but after several requests to the principal Melanie, the bill was sent. She refuses to refund me half the utility amount. I have all the emails requesting her to do so but she just ignores me.