The winner of our outstanding ethical achievement award does not have to think in food miles. When it comes to the ingredients served at St Werburghs City Farm Café in Bristol, Leona Williamson can think almost entirely in food yards. 'One of my aspirations was to source as much local food as we can,' Williamson says, 'And really you don't get much more local than this.' We are looking into a sty containing four happy, fat Gloucester Old Spot piglets, which is just across the road from the café where their meat will eventually be served, so it's hard to disagree.

Williamson, now 30, is a slight, delicate woman with a flash of auburn hair whose size belies her massive ambition. Over the past three years she has taken a small café and created from it a stunning community resource, one that succeeds on so very many levels. St Werburghs City Farm, which has been running in Bristol for 25 years, is tucked into a hilly corner of the city, surrounded by allotments, but defiantly part of the urban landscape. However, until Williamson took over the lease on the café, the farm was - as so many city farms are - little more than a petting zoo; a place where city children with little experience of rural life could get close to farm animals. Despite the name, though, nothing was actually being farmed.

The new girl, then still in her 20s, changed all that. She was determined that the café should source its food from next door. She hired a livestock manager. He quickly found a working farm with its own butchery facilities in his home village just outside Bristol who could prepare the animals for them. Now instead of merely keeping pigs, they could breed them. The day I arrive four of them, the siblings to these piglets, have been sent to slaughter. St Werburghs is not a place for sentimentality.

'We'll keep one pig to play around with,' Williamson says, 'for prime cuts and some of the offal. The rest goes for bacon and sausages. But we also run a meat club which allows locals to buy cheap pork. The moment we know we've got one of our animals coming back we phone every one up and they come and collect. Our animals come back to the community in which they were raised.' Tim Child, the livestock manager, chips in. 'I live in the village where the butchery facilities are located and I have to drive here anyway it means I can bring the meat in with me. So really we have no food miles at all.'

They do the same with the goats they rear. Chickens are kept for their eggs and they also supply their kitchen from the farm's other activities. A short walk away, for example, is a community garden planted with rocket, mustard leaves and various herbs, which is looked after by adults with learning difficulties, part of a scheme run by the farm. Williamson takes me down to have a look. It's a beautiful spot of wild terraced plots carved into what was once hard, rubble-scuffed waste ground. As we talk she squats down to pluck armfuls of rocket for the kitchen, and fills a bag with fresh sage. On our walk she also points out the allotments, which are looked after by locals. 'People who have the allotments aren't allowed to sell any of their produce,' Williamson says. 'But they are allowed to trade so we make it known that we're happy to receive what they've got and in return we give them lunch, according to how much they've provided us with.'

All of this is, of course, hugely admirable. It is a fabulous combination of a sustainable, local -community food project and the kind of urban area that so rarely gets involved with such things. What makes it so very much more than that is the irresistible quality of the food served at the café. 'When I took it over,' Williamson says, standing outside. 'It was just doing cakes and sandwiches. I wanted to do much more than that.' She leads the way inside. 'Come in to my Hobbit house,' she says with a grin. It's a good nickname. A squat, semicircle of a building, overlooking a children's play area, it was given a makeover 10 years ago by a bunch of local artisan builders called the Bristol Gnomes. They used pressed, carved and heavily varnished plywood to create a Gaudí-esque fantasy, of curling organic shaped golden wood.

This morning, as on most weekdays, it is filled with groups of parents and toddlers enjoying the burst of sunlight. There are piles of books for the kids and a few toys. Some of the food is standard café fare. They do lots of all-day breakfasts. There's bangers and mash and Welsh rarebit and burgers, from a menu that changes with the seasons every three months. But there are also daily specials. I try some of the pig-liver pâté, made with meat from one of their animals. ('We don't normally name the animals but I called that one Serrano because we wanted to try making our own ham from it,' Williamson tells me.) It's a rich, dense pâté with a gentle, sweet gaminess. After that there's a plate of traditional faggots, with a fine onion gravy and mash made from celeriac, Jerusalem artichokes and potatoes. This is serious food, at impressively reasonable prices. There's nothing on the menu over about £6. 'Food is food,' she says. 'You don't need to mess about with it, if the ingredients are good.'

In the kitchen is chef Paul Burton, 26, who recently joined the café after a career in mainstream restaurants, and who is a partner in the business. 'I wanted to come on board because it was an opportunity to work really closely with the produce,' he says. 'I love the fact that the faggot you've just eaten came from the older brother of the pigs over the road. I love the fact that I can come up with my own sausages or make chorizo. We're just doing everything right.'

Not that it's easy. There are just two of them in the kitchen and on the weekends they can send out over 250 platefuls of food. Indeed, it's clear that making the café a success is a tough and arduous business, and all of it is down to the entrepreneurial spirit and pure bloody-mindedness of Leona Williamson herself.

A Bristol native, she grew up in the streets surrounding the farm. When she was just three years old her mother walked out, and she and her brother were taken in by their childminder and her husband who, in time, became their foster parents. 'It's actually a very happy story,' Williamson says, 'because we so easily could have ended up in the care system.' Instead, she found herself in a very happy - and very foodie - family. 'My foster father is Jamaican and he ran a Jamaican restaurant in the St Paul's area.' After school she often went there to work. 'I did a lot of peeling of vegetables and squishing things,' she says. 'It's also where I got my love of goat. It's a great meat and you don't see enough of it on menus.' She clearly regards that food-orientated childhood as privileged. It's one of the reasons they run cookery classes at the café whenever they can. 'I want kids to know where food comes from and to understand how it works,' she says. Classes are cheap and some are free for families who can't afford to pay.

After school Williamson went to work as a waitress for a local Latin-American restaurant. 'They had just two branches when I started there, but soon they began expanding and I was given the job of training up all the front-of-house staff for each new launch.' It gave her a solid grounding in the restaurant business, which led her to return to college to get qualifications in training and development. 'It's a highly commercial background which I think has given me some backbone.'

Even so, she admits making the farm work has been difficult at times. 'You have to do things in little stages or you frighten people,' she explains. And some things are tricky, however hard she tries. 'The one thing I have to import is tomatoes for the breakfasts in winter. It drives me nuts. But if I take them off, the customers complain and without the customers I'm nothing.' Still, she pretty much manages to keep to her principles. They use a supplier of renewable energy for their electricity and she manages to do without heating because of the design of the building.

Now she has bigger plans, driven as much by necessity as by ambition. The fact is that keeping the prices down where she wants them, while hitting the level of quality she demands, is tough financially. 'We have made some losses,' she says. 'And clearly that can't go on forever. So what I want to do is build a deck outside for extra seating. We need bigger turnover to secure our future.' She also has plans for a planted 'living roof' which should keep the café warm in winter and cool in summer, plus there will be a rainwater run-off which they'll use to flush the loo.

However, to make all this work they will need £24,000. 'It's not a lot of money for what we want to do, but it is a lot for us.' Local resident and café regular Daddy G from Bristol band Massive Attack has agreed to do a fundraiser, and she has other ideas for how to bring in revenue. Only a fool would doubt Williamson knows how to get the job done. 'We're a social enterprise,' she says. 'We're trying to keep a community alive through our food policies. What's more there's absolutely no reason why what we've done can't be repeated elsewhere.' And it's that killer combination of determination, practicality and role model which has made her a truly deserving winner of our outstanding ethical achievement award.

· St Werburghs City Farm Café, Watercress Road, Bristol 0117 9428241