BAD CUSTOMER RESOLUTION AND SERVICE

My car was recalled by BMW Bedfordview for a possible faulty airbag. I booked my car in. I asked them to replace a few things with the airbag while it was there. Later on the job was completed and so they called me to confirm its done BUT that the car is not starting at all. They suspected it was my DME unit on my car. Bare in mind my car drove into the branch perfectly fine but they still want me to pay for something that has been broken by them, still saying I am responsible for the cost. I am a very fair person and serviced my car with them the last 4 years but making your own customer have to take the responsibility, is not on. My TRUST is broken, period.