Moreish, satisfying and cheap. These are the qualities that first drew me to mexican food when I was young and poor. I remember stepping off the grey street into a world of bright colour and explosive tastes. The food was punchy and peasanty, I washed it down with a jug of limey margarita and left filled with the spirit of fiesta.

Ever since, wherever I go, I try to find a good mexican. In New York, for instance, you can eat like a bandido three times a day without getting bored. On a recent trip I sampled huevos rancheros in a breakfast diner, then queued at a burrito bar window for a foil-wrapped tortilla at lunch. Neighbourhood restaurants doled out piquant salads and fragrant fajitas at night and I even found a nightclub serving bite-sized mexican tapas and tequila shots to manhattan foodies.

But until recently London restaurants have failed to match up, serving overpriced, under-flavoured food and watered-down margaritas. Suddenly, though, the capital is in thrall to its own miniature mexican revolution. Thomasina miers's Wahaca, serving street food in Covent Garden, has spiced up the market and a smattering of new burrito bars and restaurants are doing fierce trade.

Santo is the newest, sat edgily at the wrong end of Portobello road, beyond the Westway flyover. The restaurant spilled happily out into the street on the sunny evening I visited and the pavement diners had the dishevelled air of daytime drinkers. tequila-fuelled crowds are never conducive to dining à deux so it was a relief to discover the smart décor and sleek furnishings inside had a more sober feel. Tequila was nonetheless necessary. Ignoring the fresh pink watermelon margaritas we opted for traditional versions - upgraded to "especial" thanks to the addition of agave syrup. The cocktail had the requisite limey kick, but at £8.50 each I didn't dare grow too fond of it. The menu is divided into daytime, evening and weekend sections, the latter offering tempting brunch dishes and the daytime list very reasonably priced "bowls" and substantial mexican sandwiches called "tortas" alongside imaginitively filled burritos and quesadillas. I began to regret my choice of night-time dining.

From the starters, which included chicharron de pescado (fried fish strips with lime and hot salsa) and los tres (three different flavoured little "sopes"), we chose lemon sole ceviche and nachos. My first bite of the sole was sublime - clean, fresh and simply seasoned with lime juice, fresh herbs and tomato dice. But the second mouthful had a hit of chilli which completely overpowered the delicate fish. It came with too much avocado and an unnecessary dish of sauce so hot it made my eyeballs sweat. The fish didn't stand a chance.

Nachos came slathered in good beans, sharp cheese and guacamole that danced on the tongue. But the nachos were not the crisp corn triangles we anticipated, they were the deep-fried pastry kind and had wilted beneath the weight of toppings. mains were more successful. With the encouragement of our helpful waitress I opted for pescado entero al ajillo - a whole grilled gilthead bream. a silvery fish with a pretty face it came perfectly cooked, smeared with a smoky chilli aoli. My friend's grilled vegetables in a mole sauce was less triumphant. The veg - peppers, courgettes, asparagus - were nicely cooked and the mole an interesting combination of sweet, bitter and spicey. He liked it. to me it had the texture and flavour of a sauce made from Chri s tmas pudding. Though I missed the salsa and guacamole sides you come to expect in a mexican joint, neither were our plates overloaded with the usual heaps of cheesy beans and greasy rice, a relief on a hot night.

Sadly this meant we had room for pudding - never a good idea in a mexican restaurant. Intrigued by the advertised "goats' milk toffee" I rashly ordered the crepas de cajeta. The toffee was unremarkable and the crepe rubbery and filling. The home-made vanilla ice cream it came with would have been fine on its own.

I didn't hate Santo. Service was good and when mistakes were made - a margarita came on the rocks instead of straight up - they were rectified rapidly. The space was pleasant too, with none of the usual mexican kitsch cluttering the walls and the music tending more to jazz than mariachi.

But I didn't love it either. The food was pitched higher than necessary and basics overlooked - you don't go to mexico for fine dining but the quality of the salsa really matters. And the bill - £92 for two with only two drinks each - made my eyes water almost as much as the chilli. I'll go again but in the daytime, when I can while away the hours with a cool cocktail and a tasty, affordable credit crunch brunch.