'In my life I've met three very inspirational people who have really moved me,' says Levi Roots, standing in his yellow kitchen, counting on heavily ringed fingers. 'Bob Marley, who I played football with in Battersea Park. He was very good. He could have gone professional. Nelson Mandela, who shaped my identity and inspired me spiritually, and Peter Jones who, well, who found a celebrity in a sauce.'

The sauce is dark and runny and smells of caramelised pepper. The recipe, Roots says, is 'passed down from my grandmother. I could tell you what's in it but then of course I'd have to murder you.' After selling out at his Notting Hill Carnival jerk-chicken stand, in 2007 he took the secret sauce recipe to the BBC2 show Dragons' Den, where contestants pitch their business ideas to multimillionaire investors. He remembers waiting with a man called John, who had invented a horsewhip with a wing mirror, and William, who was looking for cash to expand a business selling clothes dryers that worked in the rain. Roots was asking for an investment of £50,000. When his name was called, he says, 'it was like I was being introduced onstage', so he climbed the stairs, dreadlocks loose in a black suit and polka-dot tie, his acoustic guitar slung on a shoulder, and sang the delights of Reggae Reggae Sauce, so called 'because it puts music in your food'. As he stood in front of the millionaires, where hundreds of flailing entrepreneurs had, variously, wept, sweated and pitched juicers that connected to the internet, he sang his gentle little song, and the millionaires began to tap their feet. 'Levi Roots,' asked Theo Paphites. 'Is that your real name?' 'No,' Roots replied. 'It's Keith.'

After tasting the jerk chicken, Peter Jones and Richard Farleigh bought 40 per cent of his Reggae Reggae business. 'Rastafari fixed it,' Roots says. 'Jah knows, the whole episode was unbelievable.' Within weeks his sauce was on the shelves of Sainsbury's. It became their fastest-selling product, outselling Heinz tomato ketchup in its first year. Fourteen months later, aged 'a proud 50', Levi Roots is publishing a Caribbean cookery book, and working on a pilot for a television show set in Jamaica, 'taking the sauce back to the source'.

Roots was born Keith Valentine Graham in Content, Jamaica. The youngest of five, he lived with his grandparents until he was 11, when his parents sent for him in south London. 'When I arrived I was a bit shocked. I had only met one white person in my life, and I couldn't read or write. I excelled at climbing trees and making slingshots. My mum would take me to Tulse Hill library, and I'd learn from Famous Five books,' he grins, at home in his Brixton flat.

The flat is obscenely tidy. Framed photographs of his seven grown-up children and four grandchildren balance on the desk and record player, and a wide-screen TV hums with 24-hour news. A rope of backstage passes hanging on a door points at his musical past - in 1998 he was nominated Best Reggae Singer at the MOBO Awards. 'Yes,' Roots pops his head around the door, 'those passes were one of the clues when I was on Through the Keyhole.' Did they guess who you were? 'Yeah, not my name, but they knew it was the sauce guy.'

Crates of Reggae Reggae Sauce and its varieties - one with apple and another with guava - are stacked beside the fridge, on top of which is a selection of rums. A door leads out to a small lawn, littered with empty sun-bleached jars of tomato juice.

We follow him out of the gate to Brixton Market, to buy ingredients for the lunch Roots is planning for his girlfriend, Timmi. 'It's a great thing to take photos of the food and people in this market,' he says, as we round a corner, 'because usually we only see cameras filming pieces about gun crime.'

The second he enters the market, Roots is surrounded. Men shout his name from across the street. 'Respect,' he replies, fist raised. Women shake his hand. A young white guy asks him to give an inspirational talk at his school. As we tramp towards the fish stall to find some sea bass, shoppers shout, 'How's Peter Jones?' 'Good, thank you, he's great!' Roots replies. He points out Reggae Reggae Sauce posters stuck in the windows of Brixton shops. 'At first, people thought the logo was too Rasta. Too Jamaican. Too black. But I thought "to hell with them". That's me.'

Back at the flat, Roots lays out the ingredients for his Seriously Special Boat Fish. He bones the sea bass by cutting along both sides of the spine then snipping it with scissors. 'This is the dish my grandmother used to make for us in Old Harbour. She'd bone the fish, then all the kids could decide what to stuff them with. Once I stuffed it with worms.' Today, for Timmi, he's using okra, ginger and onions. He holds up the spine. 'In the ghetto, we wouldn't waste anything. We'd fry the fish bone and then us kids would fight over it.' He rubs lime over the fish, and wraps it in tin foil. 'My grandma would use long thistles to sew the flesh closed. I just wrap it tightly, so it looks like a little silver boat.'

Growing up in Britain, in the 1970s, Roots says, 'I couldn't escape from being a ghetto kid.' He joined the Coxsone Sound System as a teenager, writing songs in a Brixton squat, and travelling dance halls with his records. 'I played in front of 50,000 people with James Brown. But life in Brixton back then was tense. The Sus laws [the stop-and-search law that permitted a police officer to act on suspicion] made sure of that.' Aged 15 he was sent to Pentonville prison for six months, charged with assault on a police officer. In 1986, police raided the youth club he ran, and he was sent down again for possession of drugs. 'Lying on my prison bed I did my thinking. I left behind Keith Graham, and I became Levi Roots. I decided to do good.' Keith still lurks though.

'Keith, now, is my quiet, serious side,' Roots explains, as he fries slices of plantain in vegetable oil. 'Keith is the guy you photographed in the market. He's the one who does the serious faces. Keith was the one sweating at the end of Dragons' Den, while Levi is the extrovert, the smiling guy you see on telly. But Keith is sort of famous too - on Facebook apparently, I'm king of the Keith society, over Keith Moon and Keith Chegwin.'

Timmi arrives as the plantain is served. 'I've known him since 1984. To me, he's just Levi. This,' she gestures towards the camera, and the sauce, dribbled over her fried plantain, 'this, when women get him to kiss babies in the street and people want his autograph on their bottles, is just his day job. But he's a typical local hero - it takes him an hour to cross a room.'

When the fish comes out of the oven it fills the kitchen with warm, sweet smells. They eat it with Jamaican biscuits and his Fiery Guava Dipping Sauce. 'Recently I said no to Golden Wonder,' Roots says, holding a forkful of fish. 'They wanted to work with me, but crisps are not, and never will be Caribbean. And I'm all about Caribbean food. I believe it's the sleeping giant in the British palate waiting to be awoken. This is the new Indian food. I want to make the idea of Caribbean food friendlier - I want people to stop being scared to shop in Brixton. I want them to think of Levi Roots, the grinning guy off the telly, rather than a seedy ghetto takeaway. I can't ask Peter Jones about tunes or flavours, but he will advise me on marketing and business.' At 41, Jones is the youngest Dragon, and also the richest - his telecoms, publishing and recruitment businesses have earned him an estimated fortune of £160 million. So far he hasn't ventured from his £3 million Buckinghamshire 'fun house' to Roots's Brixton estate, but, Roots promises he will, soon. 'I liked the sauce,' says Jones, 'but I liked Levi more.' Roots says he's his mentor, 'and I've been trying to dig the Rasta out of him. I've had to go deep, extremely deep, but I think... yes I believe I've found it.'

Seriously special reggae boat fish

Jamaicans are into fish and okra: 'Fish and festival is the boom.'

Serves 2

500g sea bass
¼ tsp ground black pepper
¼ tsp salt
1 tbs olive oil
1 large fresh thyme sprig
1 large spring onion, green end only, chopped
1cm piece root ginger, peeled and sliced
½ garlic clove or more if you like it, roughly chopped
1 small carrot, peeled and cut into lengthways slices
¼ onion, roughly sliced
2 okra, ends cut off and sliced lengthways in half
¼ of christophene, peeled and sliced lengthways (or 50g regular potatoes)
1 Scotch bonnet chilli, deseeded and sliced into rings
1 tsp Fiery Guava Dipping Sauce

Ask your fishmonger to scale the fish for you. Gut the fish into the boat shape, then rinse under cold running water.

Season with salt and black pepper, and place on a large sheet of foil. Brush the fish inside and out with olive oil to keep it moist. Push in the thyme, bash the spring onion to release the flavour and lay in the base of your boat. Arrange the ginger slices in the fish and in the head. Arrange the garlic on top, then arrange the carrots in the boat. Scatter over the onion and arrange the okra and christophene or potatoes inside. Finish with the chilli rings. Smother over the Fiery Guava Dipping Sauce.

Wrap the fish in the foil, and either cook at 180°C/gas 4 or barbecue for about 30 minutes until the fish and vegetables are tender.

· Levi Roots' Reggae Reggae Cookbook is out 2 June, HarperCollins, £14.99. To order a copy for £13.99 with free UK p&p go to observer.co.uk/bookshop or call 0870 836 0885