In dispute with the garage over breakdown claims

My car developed some problems after a couple of weeks in buying it, the garage has had the car now for over five weeks and said that the reason the car would not start is that they say there is petrol in the diesel and I know for a fact there is not, they say that as we have been turning the engine over and over we have burned out the starter. The car only made a clicking sound when the key was turned, and as we had been push start the car until it would not even start with a tow they say that we have damaged the ECU unit beyond repair. Could you please tel me if this is possible as I am an eighty year old disable pensioner I paid £1100 for the car in cash. Thank you