A DECADE after it was first planned, they finally built the new shopping centre. At the end of November, Safeway moved into 45,000 sq ft of purpose-built superstore. A Lo-Cost has taken over the old Safeway premises. Nearby a Waitrose is under construction. The local Marks & Spencer recently decided to expand its food shelves at the expense of clothes. And a developer has presented plans for an off-the-high-street superstore half a mile away on a scale to eclipse even the new Safeway.

Nags Head in Holloway, north London, has always been a place to buy food, especially cheap food. But now the supermarkets appear to be displaying the same herd instinct which has elsewhere created parades of building societies and estate agents. Meanwhile market forces have elbowed out a Woolworth and Jones Brothers, a department store.

So while there are fewer places to choose clothes and furniture, Holloway food shoppers can now draw the sort of distinctions normally only made by participants in Taste Test features. Whose sauce is 'nice and tomato-ey with a good texture'? Which one is 'metallic and over-sweet'? And will the opportunity to compare and contrast keep all those tills ringing and bleeping?

Some analysts believe the number of new supermarkets nationally is reaching saturation point. The Nags Head shopkeepers have been arguing that Waitrose should be the last new store, though they would favour a DIY or furniture shop on the megastore site. The fear is of an imploding market, which would leave even bigger and uglier holes than those created by the bankrupt estate agents.

New point-of-sale technology is one reason for the rush to build ever more efficient supermarkets. Computers, not human staff, do pricing, re-ordering, stock control. But how to know when enough is enough? The last big idea developers had was new offices, supposedly required by technological changes in the Eighties which had made Sixties office blocks obsolete. Now there is a 25 per cent oversupply, and property market analysts say it will be at least 15 years before demand catches up.

Jeremy Alun-Jones of Lehman Brothers, the stockbrokers, is warning clients to be cautious of the supermarkets' calls for cash to finance the massive building programme which created 78 new stores nationally last year, and is set to continue for years. 'There are areas of the country where the market is already looking saturated,' he says.

Until recently Holloway's problem was shops disappearing, not rushing in. Islington, the local council, says it has no legal power to discriminate between different sorts of retailers. It can court retail chains, offering convenient premises and purpose-built car parking, but it cannot second-guess its decisions on what the market will bear.

On the first day's trading of the new Holloway Safeway, suited figures grouped in little knots from head office, the developers and the council planning department. They discussed the take-up in the car park ('Only a quarter full on the top floor') while the town crier, jugglers, clowns and a trad jazz band tried to counter a certain bleakness created by the unlet shops.

Waitrose meanwhile, many months away from its opening, was still confident it had 'a good commercial opportunity to offer something distinctive - we can't comment on our competitors' decisions'. Sainsbury was planning to stay open next door - and the project for a megastore of up to 70,000 sq ft was still alive. Alan Headley of the developers, Landmark Projects, said: 'If the market was saturated, the superstore owners themselves wouldn't be seeking to open.'

For a while on its first morning, the new Safeway had more gawpers than shoppers. Then it became busy while Sainsbury emptied so much it seemed the food would sit around till past its sell-by date. But five days later Sainsbury was fairly busy again, and Safeway had 15-minute waits for the check-outs. The staple conversation in the queues was the perceived differences between the stores. When Argyll, the Safeway parent company, opened up the utility supermarket Lo-Cost the following week, there were takers for its characteristic special offers - on sliced white bread, West Country faggots and British sherry. So far so good - at least until the next big opening.

A supermarket company that does not open new stores will lose market share to its rivals, declares Mr Alun-Jones. He believes the high street game of musical chairs will end in tears if the current rate of investment continues, but no one chain can afford to be left standing. 'Under a free market system, the high streets are an asset for sale to the highest bidder and just now it's the supermarkets who have the cash.'