Fault in the microoven

I bought a micro oven in October 2014 start using it in June for the first time after my kitchen units were installed.the objected started to burn and I took it back to be fixed as it had a warranty after a week they informed me it was fixed to collect I did that when I got home it was still doing the same thing I had to take it back on the 16 June 2015 I wanted my refund as I have not used it and they told me it was fix of which it was not. I was told that if I had brought it back within 8 days after I bought I am frustrated wen I told them that if they don't want to give refund they must give me a new micro oven tendayi said no ,they will still fix the same micro oven that was not fixed.bad bad service will never buy there again