I know what Andy is saying, the consumer's contract is with the retailer not the manufacturer, that's the legal position so use it if necessary.
Personally I usually take the easy way out, my life is too short. Bad service? Take my business elsewhere and forget about it (if possible). Obviously not always possible, but a case of overfilled oil and possibly failure to drain/change oil and filter, do it yourself and know it's done right. Don't go back for more cr*p.