Tracy Worcester is wandering through her garden with an armful of cow parsley that she plans to arrange in the drawing room; she is railing against buying imported roses from Kenya. You might think that a marchioness would have a rather more extravagant, Elton-John attitude to flowers around the house, but Tracy Worcester takes her green beliefs extremely seriously; woe betide the unthinking guest who turns up to dinner at Badminton with a shop-bought bouquet.

But then Tracy, 47, has always been very much her own person - unafraid to stand up and air her views and to use her title to get them across. Unconventional from an early age, she was expelled from school for thumping a teacher, was modelling in Paris by the time she was 17 and then became an actress (along with her sister Rachel) under her maiden name of Ward (Cat's Eyes and Miss Scarlett in the the short-lived Cluedo). When the acting roles began to dry up she was drawn to the ecological movement, partly through her friendship with the Goldsmith family who ran the Ecologist. In 1989 she started working for Friends of the Earth - 'It gave me something to do - stuffing envelopes, sticking on stamps - and it all started from there.'

Tracy was one of the first and most committed eco warriors, famed for her ecological views and an alacrity to go on protest marches (the hunting ban, various contentious bypasses). She attracted a lot of press and quite a few brickbats (Taki attacked her in his Spectator column), but she stuck to her guns. She has been a committee member of Friends of the Earth, the Soil Association and the International Society for Ecology and Culture, and now she has branched out into making films.

However, at the forefront of her mind today is local food. A friend is organising British Food Fortnight (23 September-8 October) and Tracy is right behind it. She is vehemently opposed to food miles, albeit from the luxurious position of having her own wonderful vegetable garden. She makes a point of buying food from local shops, producers and markets, and frequently opposes the expansion plans of her local Tesco.

So today she is holding a picnic in the garden. And as part of BFF's agenda is getting cookery lessons reintroduced in schools, Tracy's youngest son, Xan (short for Alexander, 10), and his friends will be cooking some of the food.

Her two older children, Bobby, 17, and Bella, 15, are wisely keeping out of the way, although their return from boarding school is evident from the overflowing trunks in the front hall - the sports gear and trainers mingling with a couple of dishes of cat food, the family wellingtons, riding boots and a riding crop.

The Worcesters live in a beautiful but rather chaotic Cotswold stone house by the gates of the 52,000 acre Badminton estate, which one day Tracy's husband, Harry, Marquis of Worcester, will inherit from his father, the Duke of Beaufort. Harry (hugely tall and nicknamed Bunter, although Tracy calls him Harry) has not always seen eye to eye with his wife's causes. He put his foot down at recycled loo paper, the introduction of a compost straw loo in the garden and the possibility of housing a community of travellers in the corner of the estate. He keeps out of the way today as well.

When we arrive Tracy is out jogging (she runs every day, which goes some way towards explaining her whippet-thin figure) and Xan and his friend Tom, who have covered the study floor with Lego, are deputed to show us round the garden. This is a glorious mixture of quite formal, tamed spaces and rampant weeds that sometimes hide discarded children's toys. Right outside the back door are several carefully sorted recycling bins, and nearby are leftovers for either the compost or the chickens. Accompanied by Tracy's greyhound, Fern, who came from an animal rescue home in Bath, we wander through the chicken yard to the polytunnels where Tracy grows organic tomatoes, spinach, lettuce and peas. For a while she sold her excess produce at the local farmers' market but it became too overwhelming with all her other commitments, most of which now largely centre on making ecological films.

She's made one film in Poland to highlight the plight of pig farming and the illnesses it is causing in the surrounding areas, and one in Bhutan. She was caught climbing into an intensive-rearing pig farm during the Polish shoot and had to hide out for a couple of hours in a wheat field.

The nice thing about Tracy is her utter sincerity, passion in her beliefs, and her unaffectedness. Shades of her acting past come out when she's off on a rant - food globalisation and the failure of Make Poverty History are two of today's topics - she strides up and down waving her arms, although ironically she says that if she's giving an official talk she gets nervous and has to have a script.

'Global corporate capitalism is incompatible with environmental sustainability,' she says. 'I want to change the planning regulations so that people who want to leave cities and set up sustainable development communities - where they live on the land and support themselves - can do so.'

She tries to ensure that her children aren't brought up oblivious to the real world.

'We went on holiday to Kenya and visited the largest slum in the world. My children were standing in an open sewer, seeing for themselves what it's like.'

This is probably important when you have a pretty idyllic childhood running around the Badminton estate. The garden is a children's paradise: a swimming pool, flat lawn for croquet, tennis or cricket, a battered old caravan for playing in, horses and then a huge, and gloriously established old-fashioned Peter Rabbit-like vegetable garden with parterre hedges, espaliered fruit trees and loads of good things like raspberries and redcurrants.

Tracy is disarmingly frank when it comes to cooking up the picnic, saying she can't cook, and leaves it to her housekeeper Judy. By this time nine-year old Marina and her younger brothers have arrived and a straggle of children go to the hen yard to collect eggs for the cakes and to pick raspberries to fill them. The wait for the oven to do its job seems interminable, but finally everything is carried out to the orchard and the food is consumed. Fast. And if school cookery classes could be as relaxed and fun as the one in Tracy's kitchen, then British kids would have a ball.

· British Food Fortnight runs from 23 September-8 October and involves over 26,000 schools, plus pubs, shops, restaurants and organisations. For information on taking part: 020 7840 9292; info@britishfoodfortnight.co.uk Britishfoodfortnight.co.uk

Badminton picnic food

Oven-baked cheese and bacon omelette
serves 6-8

300g pancetta
2 medium-sized white onions
6 large organic eggs
5 fl oz double cream
200g strong cheddar cheese, grated
salt and freshly ground black pepper

Cook bacon until crispy. Drain on kitchen paper and put to one side.

Preheat the oven to 200°C/gas 6. Peel and chop the onion. In a frying pan heat a good knob of butter and add onions, fry until soft and transparent. In a mixing bowl whisk together the eggs and cream; add onions and bacon and three-quarters of the cheese. Place in a flan dish and sprinkle the remaining cheese on the top. Place in the oven for 30 minutes, then reduce heat to 180°C/gas 4 for a further 5-10 minutes until set.

Broccoli soup
serves 10-12

1.6kg broccoli
1 head celery, chopped
3 large leeks, chopped
1.5l homemade chicken stock
chilli powder
salt and freshly ground black pepper

In a large pan heat 4 tbs of olive oil and a knob of butter. Add celery and leeks. Cook for 5 minutes, add broccoli and stock to cover. Simmer for 10 minutes, remove from heat, add two pinches chilli powder. Blend and pour into a clean pan. Add stock to make a smooth soup, season, serve hot or cold.