Staff are back at work cleaning up and testing equipment at the Heinz factory in north Norfolk which was hit by a major blaze last week.

A joint investigation by the company and Norfolk fire chiefs is still under way to discover why an industrial fat fryer ignited last Friday teatime at the plant, on Station Road, Westwick, which employs about 200 people.

The fire is understood to have damaged the roof of a building known by workers as the old packing house.

Heinz invested more than �16m in the factory between 2006-2010 to boost productivity and efficiency savings.

No-one was hurt in the incident which saw around 90 firefighters from 10 stations at the scene late into Friday night. All 50-60 workers on site when the fire started were safely accounted for.

The blaze closed the plant, which normally operates 12-hour round-the-clock shifts making frozen potato products for the Aunt Bessie's range.

A Heinz spokesman said on Tuesday that workers had returned that day to help with the clean-up operation. 'Our focus is on resuming production as soon as it is safe to do so,' he added.

'The new production and packing lines that were part of the recent investment were not damaged by the fire although one of the original lines is damaged.'

A senior management team was in charge of every aspect of a 'full recovery plan' and, although there were stocks in store, Heinz was taking all possible action to minimise the potential impact on customers and consumers,' he added.

Stuart Sharman, full-time trade union official with USDAW, praised Heinz for the way it had treated its workforce since the incident.

'They are looking after staff in an exemplary way. Conditions and rates of pay are being maintained and they are keeping everyone fully informed,' said Mr Sharman.

'No-one can remember any incident of this sort before. Heinz's safety record is very good. I am happy with everything they have done so far. They have clearly covered all bases with regard to their staff and I've had no complaints from our members.' He said there were between 100 and 110 USDAW members at the factory.

Police blocked roads leading to the factory on Friday night and firefighters used aerial ladders to direct foam at the building housing the fat fryer.

They faced particular problems because of the 'sandwich-panel' construction of the building: thin sheets of plastic-coated steel filled with highly-combustible polyurethane foam.

David Ashworth, group manager with Norfolk Fire and Rescue Service, said such buildings were widely used throughout the food industry because they were easily cleaned but the fire had spread to the polyurethane core where it was deep-seated and extremely difficult to access. The fire was finally extinguished on Saturday morning.

The factory, which covers 11.25 acres, has three main processing lines – one for roasts, one for chips and the third for croquettes.

It was originally opened in 1948 by Alexander and J Carl Ross and was bought by Ross Foods in 1954. In 1999 the site was taken over by UB Frozen and Chilled Foods and was acquired by Heinz the following year.