It's very hard to argue after the event.
Our maintenance folks know we want to see all removed/faulty parts (apart from oil filters, which they cut open anyway - and have on occasion shown to me anyway). They have pursued warranty claims on our behalf, and got refunds.
We had the "take your business elsewhere" treatment from another engineering outfit years ago in similar circumstances. We complained to the CAA, who told us they couldn't advise us, but they investigated anyway.
The outfit's "Approval" was withdrawn soon after - I'm told that was not just because of our complaint.
We asked around about engineers, and the advice we received was that there are reasonable ones, OK-ish ones, and dreadful ones. The best advice was to get to know the firm you use, make it clear to them that you want to know what's being done and to see the parts they replace, and give them a hard time if they screw up. And pay their bill very promptly - that's good for credibility.
We're extremely happy with the one we use now.