Grow your own tomatoes Tomato blight
Blight is a devastating disease, caused by the fungus-like organism phytophora, that affects tomatoes and potatoes. It spreads by air-borne spores, so even if you don't get it one year, it can blow in the next.
The spores can only infect a plant when certain conditions are right: the leaves must be damp, the temperature above 10°C and humidity over 90%. In other words, weather that's often a feature of the great British summer.
Keeping leaves dry helps to prevent infection, so tomatoes grown in a greenhouse are less likely to be affected. Outdoors, you could try erecting tall cloches or shelters over tomato plants. Watering the soil without wetting the leaves also helps.
The only chemical treatment is a copper-based fungicide (eg Bayer Fruit & Vegetable Disease Control or Vitax Bordeaux Mixture), which will help prevent the disease if applied regularly, but won't cure it.