NCF Living complaint: Overcharged for goods

After trying to claim on the insurance I took out for our faulty sofas I had to find our original paperwork for the furniture. Upon reviewing this I discovered that at the time of purchase I was overcharged by £600. It was a manually calculated invoice and obviously human error which at the time I did not notice. I have been trying for a year to get this money refunded and have mostly been ignored. I have sent them all the paperwork to prove my claim but still nothing. I am left with no option but to either take it to a solicitor or I am considering using the BBC Watchdog programme or it’s new equivalent. I have been treated disgustingly, not even an apology. What is most annoying is that I now work in accounts and know for definite that at the time this information was entered into their accounting system they would have been aware of this as the figures don’t match and had the opportunity to do the right thing and return my money at the time but chose to keep it.