"The bonnet lock went in working and now it's not,since the repairs.Their problem not yours"

This is not strictly true. If the garage has done no work on the bonnet or lock mechanism they can have no responsibility for this. I own a garage and its incredable how often this problem of "its was ok before it came in the garage" is thrown at you.

We had a customer the other week for example whose car failed its MOT on a blowing exhaust. A new exhaust was fitted and MOT issued and 2 weeks later they returned claiming the front brakes were squealing and the battery had gone flat and needed replacing. "What are we going to do about it?"

Somehow the garage assumes responsibility for any faults on the vehicle that occur following a garage visit even though the faults are totally unrelated to work performed.

The above example is obvious to all but the most dimwitted of people that it is not a responsibility of the garage, however when work is performed that could be related to the new fault that occurs then there appears to be a "guilt by inherited association" that is used to condemn a garage trying to resolve a customers problems. (I have to say this approach tends to be adopted by owners who have bought a bad car from others (usually fleabay) and having bought on price rather than condition somehow want to apportion blame to the garage who are only offering repair solutions at a price they dont want or cant afford to pay.