Poor diagnosis

I booked my car for a service as the booking clerk advised me that the mechanic will be able to dignose the fault that I am experiencing with the car jerking.the lady Boitumelo advised that if I book it in for a service then they will do a full diagnosis as I'm booking it for a service.if I brought the car in without a service then you pay for a diagnosis.i trusted what she and the mechanic said so I brought the car in.the machanic then advised that the jerking was caused by the blocked fuel filter and that it was not changed.i had to then come again to fit it the car is still jerking.when I called the machanic he then advised me that I must bring it back again as they need to do a full diagnosis to find the fault and that they will charge me for it.why was I told that they were doing a full diagnosis for the service and then identifying that it was the fuel filter and now it's not.i don't trust Audi braamfointein.no confidence that you guys know what you doing.i was ********** and the problem was not resolved as I was promised a proper diagnosis not guess work.i will not ever take my car to you again as I have been your loyal customer.