No after sales service

I took my car for a service more than two months ago and reported a fault ( a knocking sound) when The car is put in reverse gear. The people said that they will order the new part and will let us know when it arrived. The car is still under the factory warranty. They never came back to me. I called them since then 4 times and every time they promise to look into the matter and will come back to me, they never do. I called McCarthy head office, complaints department and reported the poor service. Once again the operator promised me that the he will contact the dealer and I will be called that same day. It never happened. The last incident with head office was also 6 days ago.