The large, draughty premises are half-kitchen, half-prison canteen, with white-tiled walls, overhead metal piping, and nondescript chairs and tables

St Anne's Court, London W1. About £30 a head before drinks or service

You can't say the Goodman Restaurants group lacks ambition. The company is based in Moscow, where it ran a chain of steak restaurants, before coming to London in 2008 and opening three Goodman steakhouses in quick succession, in Mayfair, Canary Wharf and the City. It took a lot of nerve to do so. Upscale steakhouses had taken over the capital in 2008: from Hawksmoor to the Gaucho Grills, chargrilled heifer, tickled with Béarnaise or slathered with chimichurri, were the favourite meal of the macho metropolitan – and of course the financial meltdown was in full swing.

They persevered, though, and in 2011, the Goodman fellas, who are David Strauss and George Bukhov, started Burger and Lobster. Taking their cue from, among others, our own Mark Hix and his Tramshed concept, they offered London diners either a burger, a lobster or a lobster roll. Whatever you chose, it came with fries and salad and cost £20. Despite the weirdness of the choice (like offering Dover sole or a ham sandwich), diners flocked to try it, and half a dozen sister premises opened, from Soho to Harvey Nichols.

Now the Goodmen have gone fish-tastic, and opened Rex & Mariano. Rex is Rex Goldsmith, a legend in fish-wrangling circles; he has run the Chelsea Fishmonger since 2004 and is to salmon and monkfish what his namesake Sir Jimmy was to corporate raiding and libel suits. Mariano is an employee who introduced them to suppliers of seafood in Sicily.

According to the press release, the idea behind this "Pescatarian venture" is "sourcing competitively priced fish from Sicily and the UK, to bring affordable, super-fresh dishes straight to their customers". They're certainly going for a harbour-to-table vibe here. The large, draughty premises are half-kitchen, half-prison canteen, with white-tiled walls, overhead metal piping, nondescript chairs and tables – and great big wash basins, with multiple taps. They're supposedly for customers to wash fishy gunge off their fingers, thus obviating the need for tiresome finger bowls, but they look like something you'd find in a borstal gym.

In order to put nothing between kitchen and punter, they give you an iPad instead of a waiter. You scroll through carpaccios, tartares, ceviches, oyster and grill sections, sides and salads and pudding, wine and beer, tea and coffee, and when something takes your fancy, you press the icon, send the order to the kitchen and wait for it to arrive. It wasn't a difficult way to negotiate a menu – but my wife Angie, my niece Annabel and I were less struck by the clever electronica than by the ranks of serving staff, in their Breton matelot T-shirts, standing around waiting to be given something to do. When they got the chance to connect with diners, they proved to be charming and chatty.

We each had a raw-fish starter (though they're presented as sharing plates). Annabel's seabass ceviche was marinated in tiger's milk, yuzu, coriander and red onion, and was "rather too acidic for comfort". Angie's tuna tartare, cubed and folded with avocado, chilli and chive and tossed in sesame oil, was "wonderful". My salmon carpaccio, not especially thin-sliced, sat in a watery puddle of oil, lemon, cucumber, tomato and basil. It was very bland. We made a Goldilocks trio: one dish too tart, one too bland, one just right.

The bread was late and damned odd: three great hunks of focaccia, toasted and dripped with balsamic vinegar. They were so unfeasibly bulky, you had to tear off lumps, getting balsamic on your fingers. Grilled prawns were two langoustines with heads and tails in place, grilled with lemon, red chilli, parsley and olives, and were "perfectly OK if nothing special". My tuna was a solid tranche cut from a lepping-fresh 300-pounder, caught that morning; it was grilled to perfection and enlivened with a sharp pesto. A side-order of deep-fried zucchini was just fine.

Angie's grilled sabre fish wasn't a success. In case you didn't know, a sabre fish is a five-foot-long monster found in the Mediterranean, and usually has "a faint flavour of sardines". Served in a bowl in chopped-up bits with polenta and smoked paprika, it tasted horribly dry and smelt off-puttingly of smoked eel. We sent most of it back.

A simple lemon sorbet drenched with vodka was the only pudding available. This was Rex &Mariano's first week and I'm sure it'll improve greatly. They've a nice concept here – taking freshly caught fish, cooking it simply and slamming it on plates for the busy lunchtime eater – but haven't found a proper groove yet. If they're trying to create the atmosphere of a harbour eaterie in Marseilles, a welcome start would be to ditch the iPads and encourage the young staff to engage with punters like, y'know, humans.

Food ***
Ambience **
Service (machine) **
Service (human) ***

St Anne's Court, London W1. About £30 a head before drinks or (5 per cent) service