what causes tomato blight?

Blight is caused by warm humid conditions. If the tomatoes are in the ground you need to dig them up and destroy them so that the soil is not infected. If in pots keep a close watch, remove all infected leaves and if the tomatoes themselves start to show blight then again destroy, get rid of the soil in the pots too.

Blight is a fungus and the spores can spread easily in the wind from plant to plant. Once a plant has the blight, it is unlikely you will be able to get rid of it or ripen fruit, as the fruit will turn black and rot too.
If this is a problem every year with the wet and cool conditions prevalent in the UK during recent summers, try to grow your tomatoes under some form of protection like a cheap plastic tunnel. They are far less likely to get blighted if protected from the elements.
You can also spray Bordeaux mixture or another copper based fungicide over the leaves as a preventative measure if you don't mind being non organic.

Bertie - believe it or not Bordeaux mixture is considered organic!