They will unfortuately take a defensive stance to avoid any addmitance of guilt, do you have any evidence that their own specialist advised which part to get -if so that puts you in a very strong position.
It sounds like they are taking you to court for non payment of bill for work carried out, which they did complete - what this means is whilst it is perfectly true the work was completed which would put you in the wrong.
That doesnt mean you are completely left out because what you can do is start a court case of your own effectively sueing them for the cost based upon the reasons you have cited - I know someone who had to do this with a large firm of builders - who also masqueraded on the set of Rawhide