The Potato Merchant, 55 Exmouth Market, London

What could go wrong with a restaurant dedicated to the humble potato, asks Lisa Markwell.

The Potato Merchant is of the knockabout school of restauranting, where good service and precise cooking are less important than 'vibe'

When I shared an office with my great friend Nina, the highlight of our day was going to Spud U Like for lunch. There wasn't much to choose from, it's true, but that suited me – a low-level starchy-carbs addict.

Now Nina's a world-famous casting director with her own empire while Spud U Like has almost disappeared. So imagine my delight at hearing about The Potato Merchant, a new spot in London's Clerkenwell specialising in spuds in all their glory. On a recent Saturday I took Mr M and Miss T along for a late, leisurely lunch.

It is on Exmouth Market, a street dominated in culinary terms by the mighty Moro, and around the corner from the Quality Chop House, to my mind the most exciting restaurant in London right now. So… no pressure.

In fact, I suspect The Potato Merchant would rather not suffer any such comparisons. It is of the knockabout school of restauranting, where good service and precise cooking are less important than "vibes". This is a shame, as a potato-themed restaurant is a delicious idea – think of the truffled dumplings, piquant patatas bravas, shoestring fries and more that are wowing diners nationwide right now.

But back to the menu: there are nine spuddy side dishes (from Jersey Royals to dripping chips to dauphinoise), and plenty of tuber action on the mains. Think fish cakes, potato soup, tartiflette – plus steaks and pies. Don't think vegetables; there are hardly any – aside from the pots, just asparagus and lettuce.

Mr M and I share salt cod fritters, asparagus, dauphinoise, patatas bravas, and a steak-and-kidney pie. Miss T orders a sausage roll – at 14, she's a girl of simple tastes – while I force her to add lettuce, despite it coming with salad cream (sounds distinctly unpromising).

What unfolds is a slow-moving catastrophe. A pottery dish of scalding hot Spanish-y chips and a cardboard "punnet" of dauphinoise barely warm arrive together, like a terrible, terrible blind date. The flavours are OK, but dauphinoise is at its best bubbling fresh out of the oven. Like most potato dishes, it doesn't want to be sitting around. Salt-cod fritters are decent if greasy and the accompanying aioli is punchy.

Everything is on the small side (fair enough, so are the prices – from £3.50 up to £10), except the asparagus, which has a large dollop of mash beneath the handful of spears. Potato Overload Klaxon!

The sausage roll is a glazed roll spliced with two juicy, well-burnished bangers. Seems to be a decent example of the hangover brunch classic. I get a good look at it only to see dark "pellet" things in the bread and a horrible thought strikes me. It's not, it couldn't be…

I ask what we're eating, nervously. And am loftily told it's gratings of raw purple potato. Aha. Might be as well to say that before it's served, as it looks a helluva lot like mouse droppings.

Anyhoo, the biggest problem is the non-arrival of the pie. Again, a slightly less than placating explanation. "Oh yeah, that. I forgot to put the order in to the kitchen." Do we want something else? No, not really. The pie takes 12 minutes and is richly flavoured and (hurrah) piping-hot. What's it's not is a pie. Another cardboard box, with a cake liner, some stew and a postage stamp of pastry resting on top.

Despite being a carbfest, the erratic delivery and quality means we have space for pudding, and order a sweet-potato brownie and a potato-bread-and-butter pudding. Again lukewarm, with a tiny tepid puddle of custard, but both taste delicious and are imaginative uses of the ubiquitous ingredient.

This is all a big fat shame. I wanted comfort and casualness – but is it too much to ask that the right food, at the right temperature, comes at the right time? A manager (I hesitate to call him Mr Potato Head, but he seems to be running things) comes over to check on us. By then we've paid (the bill includes the late "pie" affair) and are shrugging our coats on. But people are arriving all the time, so perhaps our reaction isn't important.

Sorry, I know this is obvious, but The Potato Merchant is a Spud I Don't Like.

5/10

The Potato Merchant, 55 Exmouth Market, London EC1 tel: 020 7837 0009.£40 for two, including soft drinks

Three more specialisation sensations

Burger & lobster

This buzzing newcomer serves succulent lobster and great burgers. Go early or be prepared to queue. Four branches, see burgerandlobster.com for details

Popeseye

The best steaks in London, say fans of these zero-frills bistros, in Olympia and Putney. Well-priced wines, too. See popeseye.com for details

Tonkotsu

A Soho newcomer whose noodle dishes are a cut above a large chain beginning with "W". 63 Dean St, London W1, tel: 020 7437 0071, tonkotsu.co.uk

Reviews extracted from 'Harden's London and UK Restaurant Guides 2013', www.hardens.com