Praise be to Plan B: local businesses adapt and carry on after the Hemel Hempstead explosion

"I got a call from our operations director at around 7am. He asked me if I was sitting down. I said, 'No, I'm lying down, of course' - I was in bed. He told me to turn on the TV and the first thing I saw was Hemel Hempstead burning. The honest answer? My first reaction was, 'Oh shit'."

That was how Jeff Jamet's Sunday morning got under way last weekend. The managing director of WaverleyTBS - the wine and spirits wholesale business of brewing giant Scottish & Newcastle - was watching explosions rip through the Buncefield oil depot in Hertfordshire. The company supplies distribution centres across the south of England, and one of its two main depots is next door.

"I knew we had absolutely no injuries - nobody was there," Mr Jamet continues. "But these are the two most important weeks of the year for us. It was a nightmare scenario, the worst thing that could have happened to us."

Customers counting on the centre working at full capacity include Enterprise Inns, Pizza Hut and the Spirit pub chain. It also had more stock than usual to cope with the Christmas rush, worth around £12m.

WaverleyTBS was one of dozens of offices and warehouses damaged or destroyed when tanks used to store petrol, diesel and aviation fuel exploded. Companies affected ranged from smaller businesses to FTSE 100 giants, such as Marks & Spencer and DSG International, owner of Dixons. Mr Jamet's dire start to Sunday was mirrored up and down the country.

John Torrie, chief executive of the IT services company Steria, lives about five miles from the depot, where his firm's head office is located. "When I heard a bang that loud, it was evident something serious had happened. It was incredible. I had no idea what the noise was. Then there was a series of different explosions after that and it just kept going."

Mr Torrie put the television on and, beginning to realise what was happening, phoned the site. He spoke to a security guard, who was "extremely shaken up". Two guards and five shift workers were in the building during the first explosion and were thrown out of their seats by the force of it.

He immediately drove to the site, but it was already cordoned off. So the group - which provides IT services for clients including Lloyd's of London and Lloyds TSB - put its business contingency plan into action. By around 10am, the board had met in another office, in Sunbury, west London, and by 4pm they were able to start rebuilding customers' systems.

"The business contingency plan was evoked immediately and we recovered the situation. We had no loss of any services," says Mr Torrie.

Steria also happened to be putting together "a major public sector bid", which was due in on Friday, and the team working on the pitch had planned to spend Sunday at head office. But by 10.30am they had reconvened in the Sunbury base and were working on the bid.

For other companies, getting operations back to normal levels took longer. The head office of the FTSE 250 IT firm Northgate Information Solutions is directly next to one of the tanks and suffered massive damage.

"I was woken up at 6.20am by a telephone call from our head of security," says Northgate's chief executive, Christopher Stone. "At first, you have no idea what's going on, but it was immediately obvious that our building couldn't have got away without being damaged."

There were four workers in at the time: three suffered cuts and bruises from the flying debris and were taken to hospital. One person escaped unscathed - amazingly, considering the damage the building suffered.

Mr Stone started ringing round colleagues. After an 8am conference call, he put the company's contingency plan into action, setting up a new head office in central London and working on getting systems up and running. By Thursday, nearly all were working again; smaller problems were due to be ironed out by close of play on Friday.

Mr Stone was allowed back on the site last Thursday. Talking to The Independent on Sunday from outside his devastated office, he described the view in front of him: "A shattered building, burnt-out cars, and to my right I can see a number of collapsed and burnt-out oil tankers. It's pretty grim."

Mr Stone said his insurers have not yet condemned Northgate's site, but that he expects it to be bulldozed.

However, he added: "I'm less upset than I thought I would be. The devastation is so complete. There's none of that thought process - that it's going to take ages to get this sorted. It's over, so you just move on."

Along with Asos, the online fashion retailer, Northgate was one of the companies most badly affected by the Buncefield fire. Asos had to suspend its shares while it assessed damage to its warehouse, which is its only storage and distribution facility. It told the City last week that it could take up to a month to be fully operational again. As with WaverleyTBS, the timing for Asos - which like all retailers relies heavily on festive trading - could not have been worse, and most expect profits to suffer.

Yet Mr Jamet at WaverleyTBS is less pessimistic than he was a week ago. Loading bays, 30 trucks, the lower floor of the office block and much of the warehouse have been wrecked. But the wines and spirits were securely stored, so he believes up to 90 per cent could be unaffected.

The group also acted quickly. Staff from Cornwall to East Anglia met in its office in Staines, west London, on Sunday to assess the situation and to contact suppliers and customers. At his own request, Scottish & Newcastle's chief executive, Tony Froggatt, was given hourly briefings. Three distribution centres - in Birmingham, Dorchester in Dorset, and Dagenham, Essex - are now fulfilling Hemel Hempstead's role.

Despite the destruction, everyone involved agrees it could have been much worse. In the words of one, it was "a blessing" that the blasts did not occur mid-week, when the area would have been teeming with people. They are also quick to praise staff, suppliers and customers, who have all rallied round.

"There's been a real Dunkirk spirit," says Mr Torrie.

General criticisms have been levelled about poor communication and confusion between the local police force and council. Yet most directors liaised closely with the police as part of their business contingency plans, and speak highly of the way they handled the situation.

Long-term and the mood is likely to change. Most insist that assessing the scale of the damage and apportioning blame are responsibilities for their insurers. But the companies affected - no matter how good their contingency plans - still have urgent questions about how such a disaster was allowed to happen.

Few will openly discuss the long-term financial implications, although one executive admits there is no doubt that these will be evident in the year-end results.

Amid all the devastation, chaos and anger, however, there have been some extraordinary ironies. The British Pipeline Agency, for instance, which owns a section of the Buncefield depot, has its head office at the site - but its buildings emerged unscathed.

Perhaps the most painful irony, however, is for Steria. In a situation that you have to laugh at - because if you didn't, you would cry - the company moved into its new headquarters only a fortnight before the blasts, after finishing a major refurbishment.

"Well," sighs Mr Torrie, faced with a yet another overhaul, "at least we've got the plans."