About a week later, visiting my parents, they said they had a present for me. It was two folding garden chairs, with silver frames and white seats and back. They said they had bought them because we needed them, and so that next time they visit I won't have to take kitchen chairs onto the patio. I don't know what they cost, but probably three times as much as my green plastic ones.
Or am I just an ungrateful, spoilt cow? 
As always with narcissists, this is really a history question. What is your history of gifts with these people?
It's possible that they noticed you needed new chairs and didn't want to be cheap so they bought the more expensive ones. If they've generally given you good gifts, then they were just clueless, and you should enjoy and use the chairs. If they've always been narcissistic and very bad gift givers, then it was probably condescending.
My brother, my Nmom's first target, is a very bad gift giver. His favorite kind of gift was something "improving." He'd give me things he thought I should want, often with a lecture on how I could use the gift to be more the way he thought I should be. Narcissists often use gifts to let you know that they find you inadequate as you are. My Nmom would also do the same thing via money. She'd give me some amount and say something like: "I thought you could use this to get some more chairs so I can have somewhere nice to sit when I come over." That was demeaning and controlling AND a double bind. I had the choice between being ungrateful and spending the money on something else, or implicitly accepting her slight by doing as she told me.
Besides history, one of the best tests of narcissism is What Would A Normal Person Do (WWANPD). A normal person would not give you chairs because in our society
you do not accept someone's hospitality and then make it clear that you thought it was substandard. The most a guest can do is accomodate their own infirmities - if someone lives in a tent without chairs, an arthritic can bring a stool to sit on. A diabetic can bring appropriate food or refuse food not in keeping with their diet. In those cases it is clear that the guest is the one with the problem, not the host. Even then, everyone is embarassed unless the guest made the special needs clear in advance, discussed the accomodation with the host, and accepted whatever accomodation was offered.
As an example of what I'm talking about: My Nmom came to visit me when I was a graduate student (poor) in environmental sciences (reducing, reusing, recycling). I used jam jars as drinking glasses. They were free, they allowed me to decrease my footprint on the planet by reusing, and if I dropped one, it got recycled. That Christmas, my brother sent me a set of drinking glasses. He made it clear to me that Nmom was condescendingly amused that I was drinking out of jam jars and had told my siblings all about what a pathetic little creature I was (in keeping with the narcissistic mother's creed that she miss no opportunity to smear one child to the others). I was outraged, and I never even brought those glasses into my house. They went into the front seat of my car and thence straight to Goodwill. The only thing worse than making it clear that you find someone's hospitality substandard, is to gossip maliciously about that hospitality.
In reality (the land where Nmom never lived) the problem was my Nmom's narcissistic envy. She gossiped about the way I lived, which was completely appropriate to a graduate student in science, because she hated that I WAS a graduate student in science. I had jam jars as glasses, and a T-shirt from a far-away place I'd gone to for field research. It was the latter item that made her angry enough to smear me, not the former. It was also anger at my failure to validate her. My dad was a scientist. My Nmom was a stay-at-home-mom. I chose a path like his. She viewed that as a repudiation of herself (and we all know how much value narcissists place on their children making the exact same choices they have).
October, you did something to piss off your parents during the initial visit (something that was probably a good thing by any normal measure). So they got even with you in their own minds by denigrating what you had. Did something come up during the visit? You talked about something that contradicted their feelings or beliefs? I know - it can be nearly impossible to figure out what sets off a narcissist.

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And no, you aren't just a spoilt kid. Taking insult where insult was meant is not spoiled.
Is there some reason you let these people come to your house and demean you by the way? Maybe if they find your hospitality inadequate, the kind thing to do would be to meet at a neutral place or at their house. That's what I did with my Nmom after she came to my house and then broadcast my lifestyle to everyone I knew with a nasty twist. She never set foot in my home again.
Chris2