Nicky, looks like some sort of disease. I see your
plants aren't mulched. Tomatoes are very susceptible to soilborne diseases. Some varieties more than others. Best thing you could do to keep tomato diseases from affecting your
plants is to mulch them with something. Straw, grass clippings, even newspaper. This keeps the soilborne pathogens from splashing up onto the
plant's leaves(usually the bottom ones first) when it rains or from watering. Also, watering with drip irrigation or soaker hoses helps too. For now, just remove the bad leaves and get rid of them. Don't leave them in your garden and don't compost them or you'll just be spreading the disease around. Then think about getting some mulch. Good luck.