FAILURE TO TAKE RESPONSIBILITY of their mistake

Hi i took my car to car service city (Jet park ) for head gasket that needs to be fixed,when i sent the car there on the 25th July they commenced and fix it.Now they ve damaged the control unit (computer box)claiming the work they ve done didnt interfere with it,so i got the report from Mini service center (Auto Alpina Boksburg) stating that theres no way to work on head gasket without interfering with box,the gentleman who currently work with the car at BMW even showed me the box cables that are attached to the head gasket.now they say i need to pay it.im so furious .