Its not your fault, just your problem

I took my car for fitment of 4 tyres a couple of weeks ago. I never paid much attention to the lock nut key, but I had a puncture yesterday and took it back to Hi-Q in order to have it repaired. They then asked me for the key and I told them that it normally stays in the boot of the car. They then said it was not there and would therefore not be able to help me. I then told them that they were the last people who worked with it and they apparently didn't place it back in the boot (or anywhere) after use. When I requested that they either find it or replace it, they told me they were not to be held liable for it.