I am vaguely aware of the Sale of Goods Act (its previous incarnation but a lot of it still stems from well worn principles 19th century commercial law) after spending a few wasted but enjoyable years in the late 80s getting a law degree...but there is a world of difference between being right in law and the actual practicalities of enforcing your rights. Where more significant sums were at stake, I've been to hearings in the Small Claims (positive result generally where you are dealing with a simple debt) and also acted as an expert witness in the High Court where even more significant sums were in play. One thing that the latter teaches you is that legal action is rarely worth the candle - generally its the depth of your pockets not the soundness of your case.
I could have returned the fan at my expense and requested postage costs back and got a full refund (mind you, the Sale of Goods Act has a lot to say about remedial action - there is not often an unassailable right to return as many new car customers have found when wishing to reject a bit of a lemon - though distance selling is, I admit, a recognition that you don't always know what you are getting) had a load of hassle etc but I would not have a fan kit for my car so the path of least resistance is to fettle it yourself and get on with things. I was fitting it myself so just my time really. I've had loads of issues in the past where I was paying for the time of others to fit a non fitting part but then you're stuck the middle - "their part didn't fit" vs "yes it does, your mechanic's an idiot". I think the motor trade is pretty good (bad) at ensuring whatever happens the customer pays...
I agree we should all complain but I'm not 100% sure, for a lot of minor issues, that it ever results in any improvement...
Bob