Why do restaurants go downhill when they expand? I understand some of the problems with chains: centralised procurement takes over, the titular celeb who set it up gets handsomely pensioned off, the winning formula, whatever it might be, doesn’t work when you try to roll it out nationwide, the people who know about food and restaurants get edged out by the money men. But opening up a second restaurant, gene edited from the successful original one, and within brisk walking distance, what could possibly go wrong? Same suppliers, same chefs to supervise and train. Not a big risk, is it?

Now I was (am?) a fan of Edinburgh’s original El Cartel in Thistle Street. I marvelled at the food that came out through that impossibly cramped serving place on the ground floor and envisaged a kitchen hotter than Tabasco, either behind or below, filled with world travellers who knew genuine Mexican food and were capable of serving up a respectful representation of it.

Now the second El Cartel in Teviot Place, haunt of students, has left me wondering was the original El Cartel actually not as good as I remember, or is this one letting it down? A quick straw poll of those who accompanied me on El Cartel trips suggests the latter.

Where the original El Cartel was electric, exciting, this one is monotonous, mundane. It’s as if its vivacity has been replaced by a tick box exercise, the dutiful copying of the inspiring original dishes, gradually losing most of its sparkle in the process. Like Chinese Whispers, El Cartel’s founding message has become distorted somewhat in the telling.

Guacamole, I used to think it quite special at El Cartel, is missing several tricks. Its greenness has an unpleasantly all-pervasive onion undertow, and is, by my standards, underpowered on lime, coriander and chilli. By the way, no need to order it as a dish because it turns up over and over again in other dishes, as does tedious shredded Chinese cabbage.

Our pick of the daily specials, a vegetarian taco based on squash, sets the tone. The sweet vegetable is partnered with eye-wateringly vinegary pickled onions mixed with a brown salsa that might possibly have a connection with chocolate, but with bossy cinnamon in overdrive and enough chilli- and I like a lot of chilli- to ride rampantly over other flavours and reduce them to mere textures. It’s one of those peppery chillis that sets up residence on the back of your tongue. I take a big slug of my rather medicinal lime Margarita and am grateful for it, even though it leaves freezer burn on my lips.

Al Pastor pork ribs manage to be dull, with a lingering taste I associate with reheating. They’re in “cumin, garlic and pineapple glaze”, a furiously fiery, fruity coating that makes us sweat and clocks in too high up the sweetness scale, but yet it can’t quite cover an underlying bitterness that I’m not actively relishing. Cashew and cabbage slaw accompanies it, but its dressing is also too sweet.

We fancy a clean, refreshing ceviche, but the only fish getting this treatment is salmon, presumably farmed, which I never eat on environmental grounds. So it’s more meat, this time carne asado, flash-grilled flat iron steak, slicked with a salsa of grassy, slightly acidic Arbol chillis, avocado, spring onion.

The tacos here are flexibly flabby, irregularly formed, hand pressed in-house daily using proper Masa Harina corn meal. They’re one of the best things about El Cartel. Duck carnitas are probably the star dish, fatty, crisp-skinned mouthfuls lightened with pineapple, brightened by hot Jalapeño chillis, everything calmed down by an emollient pecan salsa.

On paper, this second El Cartel definitely outshines the original one. More space, an eye-catching giant mural, which looks like an Aztec wrestling an armadillo, bottles back-lit and lined up behind three stain glass windows that echo the Gothic architecture of the old medical school opposite. It takes bookings. But cramped and cobbled together as it is, I miss the original El Cartel. I leave this one sticky-fingered, with my mouth on fire. I’m not getting the same buzz as before, that subtlety of flavour. It’s as if the impresario left before the apprentices learned all the tricks.

El Cartel, 15-16 Teviot Place, Edinburgh 0131 370 8189

Food: 7/10

Decor: 7/10

Service: 7/10

Value: 7/10