Misleading diagnostic

My Petrol car was accidently topped up with Diesel, then could not start. Ecu was showing all sorts of sensors faults. I took the ECU for testing to ECU express Bedfordview they said the ECU unit was badly damaged and will require repair to the tune of 10K.. But a mechanic advised draining the tank and change spark plugs, I did so then car was running again with same old ECU they said was badly damaged. These are some of error they said they found: 1* No traces of foreign fluids intrusion on the PCB module. 2* PCB hardware circuits and components are damaged due internal short circuit to ground. Missing ignition voltage outputs. Abnormal circuit resistance and response. 3* Power input and outputs circuits tested faulty. Missing digital command data outputs. I conclude that the fault 2 in particular was a lie, what were they going to fix if I paid the 10K? Besides, even if I paid the 10K, the car would not start until the spark plugs were changed. I don't recommend them for testing your ECU.