Suggest you follow up any conversations with emails/letters etc. that show who said what and what was agreed, or not, so if the worst happens you have something concrete to work with.

Been there, done that, massive hassle, lesson learned.

It's what makes me keep going back to the places that look after me. They are rarely the cheapest but I know what I'm paying for. For instance I've been going to the same Motor Factor the best part of 3 decades even though I've moved house. Mad? Possibly. But they've always come up trumps and I've never had something that has caused any sort of "discussion" about it's quality. If it's duff they change it quickly and free.