I agree, it would be the garages fault.
I have no legal expertise, but my Dad is an electrician and has a bad experience under his belt regarding this which luckily the firm he was working for was liable for.
He was recently asked to quote for a job (fitting some sockets in a new-build) and the home owner wanted to save some money by supplying his own parts. Of course he said a very firm NO. As a professional how do you know where the parts are from, what quality they are, if they are suitable for purpose etc? The cheapest items on the shelf are usually the cheapest for a reason - crap quality.
The other semi-related example I can think of is food prep.
My ex employer removed all ovens, toasters, microwaves etc (they were essential for the night staff as well) because of a case of food poisoning which the canteen franchise ended up being held responsible for.