At the newly opened Las Iguanas in Glasgow - the 34th restaurant in a chain - it took me longer than usual to clock just how abysmal the food actually was.

I was so temporarily intoxicated by the style and design of its interior, I would have settled for average food. In retrospect, I blame myself for not having checked out this chain in advance. When later I mentioned it to a friend in London, she was amazed that I had even given it the time of day. "I could have told you that it was rubbish", she said.

Apparently, Las Iguanas started out small in Bristol in 1991. In the company's version of its history, it "grew gradually and stayed independent". This chain intends to "bring more of that Latin loving to a town near you soon!". After my meal, that feels like a threat not a promise. I'm definitely not ­feeling the love. Las Iguanas strikes me as a parable on the perils of chain dining.

Las Iguanas promises "native recipes and authentic cooking techniques", the "freshest food", and "highest quality ingredients". It makes "over 30 salsas and sauces by hand each day". There's a mealy-mouthed attempt at being right-on. "We try to be GM-free", it chirps, and makes a virtue out of paying staff "at or above the minimum wage before any tips" as if that was highly progressive, rather than simply decent business practice. Prominent mentions of organic sugar cane and Rainforest Alliance coffee put a green halo above the menu.

Like this marketing pitch, the menu leaves lots of wiggle room. Its white fish (species not stated) is "sustainable", but there's no mention of any certificating body that audits this claim. Signally, nothing is said about the provenance of the chicken, a striking omission, given that the menu is stiff with it. Enquiring if it was free-range, I was told that it isn't.

Everything I tasted promised much but turned out to be a crashing disappointment. You couldn't taste any smoky chipotle in the tomato (said to be oven-dried) salsa, and even rocket-strength chilli could not have masked the stink of the chipotle whitebait. These fish were so rankly high, we had to exile them to the far side of the table. Oily fish has to be impeccably fresh or it is repellent. It was no surprise that the flabby batter on our "spectacular" fish taco was soggy under its deeply ordinary, supermarket deli counter-standard salsa. The fish itself was crumbly and flavour-challenged, fine for the cat but not a lot else.

Las Iguanas brandishes its veggie/vegan menu. From it, Bahia moqueca reminded me of the worst sort of vegetarian ready meal. A gloopy, soupy pile-up of vegetables, it was served with darkly fried plantain that reeked of used oil, rice of the firm, parboiled/ boil-in-a-bag-type, toasted coconut that smelled of almost nothing (that's an achievement with naturally fragrant coconut) and a foul, mouth-mugging raw onion salsa. Barbecue & jalapeno shredded steak, dumped on a pile of tacos and bland cheese, shone like a cook-in sauce. Cassava chips looked like South America's answer to McCains: uniform and factory-formed.

Our waitress was right to warn us about the Guyanese pepperpot, although her concern was its piquancy. Actually, it was about as fiery as a newborn baby, without any hint of the promised, highly distinctive, Scotch bonnet peppers. I checked that this dish had indeed been made with "slow-braised mutton" as advertised, and was assured that it was, and furthermore from a Scottish butcher who also supplied other Las Iguanas. All I know is that it was the least mutton-like mutton I have ever encountered. Stringy, dry, and lean to a fault, in a blind tasting I would have taken it for bad beef. Swathed in its glossy, thick gravy, with similar dire accompaniments as the vegetable curry, it was a rotten way to spend £15.

Look, I love the Latin America in the fifties vibe, the geometric tiles, the zig-zag metalwork, and I'd happily give house space to one of those funky orange settees, but the kitchen needs to be placed in special measures.