This is pathetic - if its that apparent to you on collecting the car then they should easily be able to hear/feel the fault for themselves - the issue is probably the fact they don't know where to start so it a case of engine out, change something, put it all back and trial.

If it was really apparent when you collected the car they shouldn't have told you it was fixed and ready for collection. There is an element of them not wanting to carry on with the investigations in all this, as they need to know who will be paying for their time - the warranty from the factory will cover the time, but if its an extended one they might not and the OPC are then left trying to get you to pay for their time.

I'd at least run it round to an indi to hear the fault and offer some possibilities.