After almost 40 years, PizzaExpress has taken a long, hard look at its menu and the result of these cogitations is billed as the most significant change to its line-up of pizzas since its opening in 1965.

A total of 10 "new dishes" has been introduced. Now, you can choose from appetisers such as olives, or bruschetta with Parma ham, rocket and cherry tomatoes or mozzarella and tomato salad. A Frutti di Mare pizza, as well as two other varieties, now appears on its main course menu. There is a new salad and an amaretto and mascarpone pudding. The menus come in four new covers, designed by the winners of the PizzaExpress "Prospects" drawing prize.

But does all this a menu makeover make? And should not these dishes have been on the menu years ago? In fact, events behind the scenes are likely to steal any limelight poor old PizzaExpress might have attracted for its facelift. The private finance company that took it off the stock market has just bought its closest rival, Ask, a chain of pizzerias which also offers a number of pasta dishes. If PizzaExpress seems a little battle-weary now, being half of a giant restaurant company is unlikely to help build morale.

When founder Peter Boizot opened the doors of his first Italian-style eatery in Soho, he brought Italy's most portable food to Britain. And according to the economics of restauration, its most profitable. The number grew gradually into what he preferred to call a necklace, not a chain, of arty and absolutely reliable restaurants, until the company floated in 1993.

PizzaExpress, with its hand-whirled dough-balls, its garlic toppings and oversized pepper grinders, was the perfect fodder for the Habitat-furnished homeowner and family, with its emphasis on design, art, and Venice-saving charitable donations. But a recently chequered corporate history, huge expansion and greater competition has devalued it in the eyes of investors and customers.

The "necklace" grew at the rate of 30 new restaurants a year, and now totals 308. The profits may be up - last Christmas, under its owners of just a year, they rose 7 per cent year on year - but what once seemed like manna to the middle classes lost its edge a few years back. What was once refreshingly tasteful and straightforward became a fallback for those who could not quite sink to the depths of McDonald's.

There is no dodging the Marks & Spencer analogy here. The institutions we loved for their quality, reliability and even design integrity, we now take for granted and neglect. Instead, we are lured by newcomers that stole their ideas and repackaged them. It is the pioneer's problem. The thin, crusty pizza frontier has been pushed ever forward, so that even outside London you are rarely more than an olive-stone's throw from a plate overflowing with the crispiest dough and a topping at least as redolent of Italy as Campari or espresso. PizzaExpress looks middle of the road while competitors Ask, Est Est Est, Zizzi and Strada have crept up in the overtaking lane.

Strada, started by the former PizzaExpress boss Luke Johnson, has 16 branches. Ask has 155, the latest a stunning looking building in - and why not? - Worthing. Its group turnover was up 13 per cent on the previous six months, to £51.2m at the end of last June, just when PizzaExpress's were falling. Last year it opened 30 branches.

Zizzi (same owners as Ask, but tonier in its rustic way), with more than 50 branches nationwide, and Strada make a big deal of the wood-fired ovens they have but PizzaExpress does not. And for those who want pineapple, deep-pan and the gunkier, junkier type of pizza experience, the country is covered by Pizza Hut and its stuffed crusts for a cheaper fill. The Time Out Cheap Eats in London guide, published this month, ranks PizzaExpress behind Strada.

Never mind the quality, there is also the vexed issue of size. The rumour that its pizzas had become smaller reached such a pitch that everyone believed it. PizzaExpress denied it, though nobody was sued for saying so. Its defence, that other pizzerias send out bigger ones and we had all become greedier, rings true. We live in supersizing times. But last year PizzaExpress upped its pizzas from 10 to 11 inches.

It had already tried advertising. For the first and only time just over two years ago, the poster campaign of Seventies party scenes - if you wait long enough everything comes back into fashion - failed to turn round its failing fortunes. Retro irony works only if you are selling something new, not trying to revive what simply dated. PizzaExpress lost its appeal not because it changed but because it did not. The changes to the menu may be too little, too late.

True, PizzaExpress now has 22 pizzas (from £4.95 for a Margherita to £7.95 for the new Parmense or the Pollo ad Astra, a cajun chicken topping that dates from 2002). This compares with Strada's 14 (from £5.95 to £8.95) which are 32cm (14.5 inches [note PizzaExpress has not gone metric]). But Strada has more pastas and grills. Zizzi has 15 pizzas, from £5.15 to £7.60 and a dozen pasta dishes to PizzaExpress's four. It is not just pizzas that it is competing with. Hence the extra salads, both of them. There are other sleek, funky and less dough-centred, family-friendly chains such as Wagamama, Carluccio's and Yo! Sushi, to contend with.

But let us be fair. PizzaExpress has spent £20m refurbishing its restaurants over the past two years. The Brighton branch is one of them. On a half-term lunchtime it is not full, or as resoundingly underpopulated as the vast white and glass spaces of some of the newer London branches. No white-tiled floor and op art here. Instead it is earthier toned brown tables, terracotta walls and chairs and huge moody photos of the sea.

The effect is warmer and more welcoming and rather like a Zizzi, but without the wall made of logs that the opposition has to highlight its wood-oven advantage. Also brown and even bigger than we remembered was the inevitable pepper grinder. Some things, reassuringly, never change.

Actually, most. There are all the pizzas we can recite off by heart, the Fiorentina, Giardiniera and Veneziana with olives, capers, onions, pine kernels and sultanas, and a 25p donation to the Venice in Peril fund which always gave PizzaExpress its highbrow credentials. ("I've had so many I feel I've personally saved the Bridge of Sighs," an accompanying PizzaExpress defector says.)

The children want only Margheritas, but - economy tip for parents - the extra £1.35 for extra pepperoni is a cheaper way of upgrading than ordering an American Hot for £7.80. The Fruitti di Mare is distinctly marine-flavoured with cockles and mussels and squid adequately and deliciously distributed over the garlicky tomato base.

And although the asparagus and Parma ham on the Parmense was welcome, what was most memorable about it was the overcooked egg which nobody fancied. A case of gilding the lily-like combination of Parma ham and asparagus with unwanted yolk.

The pancetta salad also failed to achieve the simplicity that makes real Italian food so desirable. The combination of Italian bacon, gorgonzola and pears, especially the latter two, is a fine one. But the pears were slivers, the bulk of it was made up with green salad and the red pepper was unnecessary.

We threw in a Salade Nicoise and were reminded that iceberg lettuce has no place in a Mediterranean salad, however much tinned tuna you throw in.

The place is pleasant enough. Cheap enough, too. Even so, PizzaExpress seems in peril. It will take more than a touch of collagen plumping up the menu to stop it being left behind by the younger pizza faces.