Panmure Gordon's analysts now value Primark at at least £3.1bn, based on the fact that it is about to expand significantly through the addition of almost 50 new shops previously owned by Littlewoods and Allders.

Primark, which trades as Penneys in the Republic, will open almost one new shop a week between this spring and early 2007. The new shops will expand the chain's selling space by about 60% over the next 15 months.

As the new shops enter the network, average shop size within the chain grows dramatically, so that by next year the average Primark will be 25,000 square feet compared to an average size of just 15,000 square feet in the year 2000.

Typically, as outlets increase in size, the sales average per square foot falls, but so far this has not happened with Primark.

The company's biggest shop in Manchester has 100,000 square feet of retail space and Panmure Gordon's experts reckon it is currently generating sales of £750 per square foot. The Manchester shop is overtrading, according to the note, but Primark is likely to expand it by adding another 50,000 square feet of space within the next two years. Apart from the addition of newly-acquired sites, Primark is also extending many of its existing properties.

The chain's Irish management has also had a subtle shift in its attitude to sites. Up to now, as it was building the Primark brand in the UK, the company went for prime high-street locations but a couple of its new shops are just off the main drag, suggesting that management is now confident that the brand and the product offer is strong enough to draw people to it.

It is estimated that it takes the company just six weeks from seeing a trend on the catwalk to replicating it instore. About 30% of its products come from China, with other locations in the Far East supplying about 20% of the firm's total output. India and the Middle East supply 25% of total stock and eastern Europe and Turkey together chip in a similar amount.

Shop managers can tailor their offering to the local market and each manager has direct and regular contact with Primark boss Arthur Ryan. When buying mistakes happen, the problem lines are quickly discounted to clear them out and generate space for new ranges.

Primark spends virtually nothing on national advertising, preferring just to promote new shop openings locally and let coverage in the fashion pages and women's magazines generate word of mouth.

Despite its huge growth in Britain in recent years, there are still big cities such as Liverpool and Edinburgh that do not have a Primark shop. Even with the growth spurt expected over the next 15 months, Primark will still be a relatively small player in British retailing.

By the end of next year it should have 126 stores in the UK, but Panmure Gordon's analysts reckon the chain could easily have 200 stores in the UK.

The first Penneys shop was opened in Dublin by Arthur Ryan and Paddy Prior in 1969; the company adopted the Primark name in the UK due to legal issues with US retailer JC Penney.

Thirty-six years later, the chain is still run from a small office in Dublin, and while Irish streets are increasingly dominated by foreign retailers, it is rather heartening to see Penneys continuing its march across Britain.