In answer to your first question i would agree with Phil H, don't lose your cool and stand your ground. I would also suggest knowing your legal position, if you can prove that the fault wasn't there when you sold the car then they really have no leg to stand on. If, as you say you've also offered a warranty then those people are exactly as you suggest 'full on pyschos' and any normal person looking at the cold facts would see you are not in the wrong so don't let them make you feel like you are. If they are aggressive and you are feeling threatened then call the police.