It sucks I know but I'm with George on this one, but I pray for your sake that we're wrong!
A large majority of garages won't give a guarantee when fitting customer supplied parts, although they should have told you this when you said you'd buy the parts..
I was going to buy the cambelt kit for my derv to save a few quid, but decided it would be best to let the garage source the bits so I could hold them 100% accountable.. Hopefully I'll never have to, but it's worth the extra £40 or so of 'insurance' (if you wanna call it that!)
I for example supplied some new rear 180 calipers to go on the rear of the derv after the original ones seized, well these ones have just gone too - I'm not going to even try call the garage as I knew the risk when I supplied the parts
Will