The Fountain Fortnum & Mason, Piccadilly, London

The Fountain restaurant in Fortnum & Mason is a bit of a mystery. Not the place itself, a pastel palace of muted good taste. What's odd is that a restaurant set within what is arguably the world's finest and most famous food shop should be so dire.

To enter Fortnum's from Piccadilly is to be transported into a soft-carpeted, sugar-coated fantasy, where nothing bad can happen – with the possible exception of an insulin-induced coma. Pretty boxes of Easter chocolates are everywhere you look, while china pots of jam wait with traditional shortbread and F&M branded boxes of Earl Grey loose-leaf tea to be purchased and wrapped up for granny. Easter must be one of the company's busiest weekends.

It's with this in mind that I check out The Fountain. I had high hopes of this English institution and could imagine bringing my family here for a slap-up celebratory meal. Deborah and I are amused to find our presence in The Fountain, one of five dining options in the shop, brings the average age down by about 20 years. There's a promotional video for The Fountain on F&M's website, in which giddy young blondes sip champagne at the bar and nibble salads. In reality, it's tables of Raine Spencer lookalikes with meringue hair-dos and equally frou-frou plates of food.

It's good to feel young, and it goes some way to offsetting my irritation at being told, on booking, that the 7.30pm time I request is not available. Can I come at 7.45pm? No problem, but when I arrive bang on time, the place is less than half full.

We start with two classics from a menu that has little other than classics – and there's nothing wrong with that, provided that they're done well. The problem is, they aren't. Deborah's twice-baked goat's cheese soufflé is on the meagre side, and bland. My smoked salmon, by contrast, is offputtingly enormous – the pile of fish served with a not-juicy-enough lemon half and some stale bread. The bread counter is less than two minutes' walk, for goodness sake.

Main courses aren't any better. Grilled pollack with cauliflower purée, savoy cabbage and bacon (£13) is at once too salty, too limp and resembles the kind of dish that fails to get a contestant through to the quarter finals of Masterchef. But it's Deborah's 28-day aged rib eye steak with béarnaise sauce and hand-cut chips that's the real howler. At £24.50, we expect a plump, soft and flavoursome tranche of beef. This is greyish, tough and with an inexcusable lump of fat in its centre. "It looks like it's from a Tesco value pack," says Deb. "Nothing against Tesco value packs, but I can get that at home, and often do."

Our waiter takes away our plates with no discernible sense of surprise at the uneaten food – either he's as good an actor as server, or he doesn't care (and he did bring the pustulant steak out in the first place. Others might have said, "Hang on, that doesn't look like a £20 bit of meat").

The attendant guitarist playing Muzak isn't quite enough to distract from the woeful quality of this meal. Even pudding – from the place that does peerless cakes and fancies round front – disappoints. Baked Alaska is frighteningly sweet and stone cold. Yes, I know it's ice-cream but the magic is in the contrast between the frozen interior and the crisp, hot meringue exterior. All I note about my vanilla brûlée with rhubarb compote and shortbread is "OK".

By 10.15pm the place is deserted, and staff are laying tables for breakfast around us. I'm in no mood to loiter but fresh mint tea seems to be taking some time to assemble. The door to Jermyn Street is flung open and a family of American tourists enter (we know they're tourists as they have the two trademarks: a guide book and petulant teenager). They order the rib eye. "That's typical," states Deborah mournfully as we exit. Perhaps we should have warned them. Our waiter had sighed "Now we'll be here till midnight" as they arrived, so it might have been a kindness to everyone to steer them towards the nearby Wolseley.

If I was feeling uncharitable I might think this place is set up to fleece unsuspecting ladies up in town for the day and tourists lured in by the Royal warrants. I've had a perfectly serviceable breakfast here in the past, so perhaps the kitchen was having a bad day. But really, my advice would be to spend your money on Fortnum's chocolate, by all means, but you'd get a better value meal at the Pret a Manger across the road.

11/20

Scores: 1-9 stay home and cook, 10-11 needs help, 12 ok, 13 pleasant enough, 14 good, 15 very good, 16 capable of greatness, 17 special, can't wait to go back, 18 highly honourable, 19 unique and memorable, 20 as good as it gets

The Fountain Fortnum & Mason, Piccadilly, London W1, tel: 0845 602 5694 Mon-Sat 7.30am-11pm; Sunday jazz brunch 11am-3pm; Dinner for two without wine: £100

More British institutions

English's

29-31 East St, Brighton, tel: 01273 327 980

A new chef has improved the food at this fadedly splendid Victorian institution and the seafood can be amazing; average results, however, are still not unknown.

The George Hotel

71 St Martins, Stamford, Lincs, tel: 01780 750 750

Grand, but not too formal, this famous coaching inn is a bastion of traditional English food and good old-style service.

Mr Thomas's Chop House

52 Cross St, Manchester, tel: 0161 832 2245

Still a top choice for delicious, if calorific, British food, this remains a special place for many Mancunians, however, there's a slight feeling among sceptical reporters that it's living off its reputation.

Reviews extracted from ‘Harden’s London and UK Restaurant Guides 2010’. www.hardens.com