Dont know about anyone else but £120 is fair bit of money fella, so definitely contest it & explain vandals dont pull of underside bits of the car do they, what do they do bring a trolley jack with them, lol, seriously mate go back to them & show them the clean break & explain again that it just doesnt happen, I'd say a screw came loose it fell down & then snapped of when driving, has been known to happen number of times especially after underside has had work on or accessed by the garage & havnt done it up properly