Unhappy for service from Midas Cleary Park Port Elizabeth

I bought a coilpack for my Corsa ,could be the 2nd last week or the last week of November there,fitted it the car drove fine,now 3 weeks later the same problem existed which we check the coil pack again it was faulty because it overheated ,the mechanic tried my friends coil in the car and tested it and the car drove perfectly with it,so I took the new coil back to Midas explaining them when we cleaned the car we threw away all papers in the car including the slip not thinking that the coil that's supposed to last us atleast 5 years will give in so quick,the sales assistant knows us and he remembered we bought it there but refuses to assist us saying we must have the proof of payment and that it is electrical and they can't refund us or give us a new one,upon which I aske if they can't get the duplicate copy of the receipt on their system because on the box it says 12 months warranty,upon which they said there is no way they have any back ups to trace the receipt which to my suprise was very surprising because how can a big shop that exists so many years don't have back up,I mean I paid R790 for the part and to just loose it like that I mean it's not fair towards me as a consumer for them to sell faulty parts and refusing to assist us,they even called the area manager upon which he also said there is no way they can assist us. Is it possible that in any way you can assist me by them replacing and giving me a new part as this is my only reliable transport I have seeing that I can't afford to buy a new car now,so now my car is standing ,I don't want my money back all I want if they can just replace the part and send in the faulty one they sold to me.