It must be one of the best-kept secrets in the Fens. British Sugar's flagship beet factory in west Norfolk is also the country's tomato capital.

It must be one of the best-kept secrets in the Fens.

British Sugar's flagship beet factory in west Norfolk is also the country's tomato capital.

Behind the seven towering silos of the Wissington sugar factory near Downham Market, millions of classic tomatoes are grown every year in a massive greenhouse spanning acres of glass.

And from next spring, an area of computer-controlled glass large enough to cover more than 15 Carrow Road pitches will be helping to meet the surging demand for British Sugar's home-grown tomatoes.

An army of unpaid workers - hundreds of bumblebees - helps to produce an estimated 34 million classic round tomatoes every year between March and November.

Now, British Sugar has announced plans to double production with a £6m investment in a second climate-controlled greenhouse at its Cornerways fresh- fruit business and employ another 20 staff.

Norfolk, which is already the country's tomato capital, will be helping to reduce imports, said the plant's manager Nigel Bartle.

Since the company built the first computer-controlled greenhouse complex behind Europe's largest beet-sugar factory at Wissington in 2000, sales have mushroomed.

The “green” complex will more than double in size to about 27 acres by next spring using the surplus heat, water and “greenhouse gas” or carbon dioxide generated by the sugar factory.

Mr Bartle said that about 2,500 bumble-bees work alongside his regular team of 30 staff, who pick, pack and care for the growing tomato plants. “We're already the largest producer of classic round tomatoes in the country but we just can't keep pace with the demand,” he added.

Cornerways was started on land at the back of the beet factory and will be adjacent to British Sugar's other “green” plant - producing biobutanol from surplus beet crops for road fuel.

The tomato plant makes uses of heat and recycles carbon dioxide from the company's flagship sugar factory, which processes more than a quarter of the nation's beet crop.

Water used to wash the sugar beet delivered to the factory carries vital nutrients from Norfolk's soils and is reused to irrigate the tomato plants.

Wissington, which runs a combined heat and power plant, is now able to recycle more than the total CO2 (carbon dioxide) or “greenhouse gases” to feed the tomato plants.

The use of low-grade or so-called waste heat also warms the network of more than 70 miles of pipes in the glasshouses.

Mr Bartle, a member of the Tomato Growers' Association, said that the investment will make a sizeable contribution by reducing the current level of imports running at 83pc of consumption. “There is growing interest in local production and sourcing and we've not been able to keep pace with demand,” he added.

The greenhouse, which uses climate-controlled computers, aims to keep as near a constant temperature of 70F or 21C for the 200,000 plants by raising and lowering blinds and vents.

“The decision was taken to locate the greenhouse next to the Wissington factory in order to make optimum use of the available energy. We also have very clear skies in Norfolk which provide plenty of sunlight and that is absolutely essential,” said Mr Bartle.

British Sugar's chief executive Mark Carr said: “By continuing to develop the usage of low-grade heat and carbon dioxide which would otherwise be lost to the atmosphere, we are able to improve the overall cost effectiveness of our Wissington operation and reduce our environmental impact still further.”

“The additional area will also allow us to support the customers who have worked with us over the past six years in meeting the increasing demand for 'produce with provenance'.”

The Cornerways plant, designed with possible expansion in mind, normally crops from the middle of March through to November.

“We're also able to collect rainfall from the roofs of our greenhouses which is collected in a lagoon and then pumped to feed the growing plants,” he said. “We had so much rainfall in August that our reservoir was full, so we've plenty of water to irrigate the tomato plants,” said Mr Bartle, who explained that the plants are fed a carefully-balanced diet.

However, tomato plants do not like to stand in the wet at night, so water is circulated. “In this way, we provide the best growing conditions to produce top-quality tomatoes,” added Mr Bartle.