Unwilling to refund an overpayment

We are a very small general dealer company. We entered into a contract with Boland Pest Control in April 2015 for services on behalf of a client. There was an initial fee due and then 8 services to be done over the next year, ending in March 2016. The initial fee and first service were payable upfront. The rest was only to be paid as each service became due, prior to delivery. <br> <br> Our order clerk mistakenly paid the full amount in April 2015, a mistake which has a huge impact on our cashflow. We immediately sent a letter to Boland Pest Control explaining the mistake and requested a refund for the over-payment. We realized it was our mistake so suggested they keep the money for services up to August 2015 and only refund us for the last 5 services. <br> <br> After numerous and constant phone calls and emails since then, and after excuse after excuse, promise after promise, they did make an offer on the 13th July 2015 to pay back the last 5 services - over 4 months! This meant final refund in October 2015 whereas our initial payment was made in April 2015! We rejected that proposal on the 14th July 2015 and have heard nothing since. It seems that they have no intention of refunding anything!<br> <br> <br>