A century-old family firm of agricultural engineers has invested in a £6m factory upgrade to keep pace with growing demand from farmers wanting to spread organic manure.

GT Bunning and Sons, founded at Gressenhall near Dereham in 1906, makes muck spreaders which are sold in the UK and exported to places including Australia, Canada, South Africa and the Far East.

And even before rising gas prices sent current fertiliser costs soaring, the company saw demand rising as farmers across the world sought natural alternatives to artificial chemicals.

The new factory includes a more efficient £1.3m paint line to remove "bottlenecks" between the blast shed, prep room and painting room, before the parts are taken for final assembly.

Its annual output has leapt from about 300 machines to 550 last year - 75pc of which were agricultural spreaders, and the rest being its other product of pallet changers for the food, warehousing and pharmaceutical industries.

Director Robert Bunning is one of the fourth generation of the Bunning family now running the company, including his cousin Sarah who is managing director.

He said the investment means the firm now has the capacity to build 1,000 machines per year to meet growing demand.

"With the prices of fertilisers now, people are now realising the value of the muck they have got sitting there in the corner of the field," he said.

"The demand for spreaders has gone through the roof. People are contacting us who have never had a manure spreader before - they have managed to get some manure from a neighbour and they now need to spread it accurately on their fields."

Sales director Chris Druce added: "I would say perhaps 15-20pc of the extra demand is because of people looking to spread manure themselves for the first time.

"Quite a lot of farmers are buying their own spreader now where they would have had a contractor or hired a machine previously.

"Some of the other growth can be attributed to the change in farming practices like we see in Australia, where farmers are actively putting up chicken sheds or getting into livestock farming to get themselves some organic manure to spread.

"So the demand for muck-spreaders in Australia has gone mad because they are all trying to improve their soil with organic material.

"There is also a huge demand here in the UK to start bringing up the organic content in the soil. I was speaking to someone the other day who said there just isn't enough muck to go around on the farms."

Mr Bunning said the firm was founded in 1906 when his great grandfather Robert Bunning bought the local blacksmith's forge, and it evolved from shoeing horses to repairing farm machinery before it became a manufacturer in the 1950s.

Today, modern technology at the new factory includes a £400,000 press brake, which folds steel panels, and a £900,000 laser cutter.

Each spreading machine has more than 1,000 parts and takes about 80 man-hours to build, spending about two weeks on the production line.

Mr Bunning said the company had been able to continue working through the Covid pandemic, and post-Brexit supply chain delays had also not proved a major problem, as the company holds large volumes of imported parts in stock.

He said the biggest issue has been a four-fold increase in the price of steel which has forced up the cost of production.

Turnover increased by 20pc to £17.6m in 2021, and the company is expecting similar growth in production and revenue in 2022.

And the 100-strong workforce is also growing, as loyal employees with as much as 40 years' experience are being joined by apprentices starting their engineering careers, training on every aspect of the firm's manufacturing.

Mr Bunning said: "I am proud of what the family has achieved so far, and I hope this generation and the management team we have got together can copy what they have done before us."