I have a photo of blight on my photo page, page 3 bottom row. Have a look at it and if it is blight then my advise is to destroy the infected plant/s and burn/bin them. Don't put them in your compost heap as the blight can survive for a few years in the soil.

I agree with Ian, it can also affect your potatoes.

If the tomatoes are turning brown and falling off follow the advice above. Do as Ian suggests and if you were growing them outdoors do not grow in the same area for at least 4 years - better long, don't grow potatoes there either! If you were growing them in the greenhouse in addition to destroying the plant you need to bin the soil as that will be infected to. Don't put it in your compost but into the wheelie bin. It would also be a good idea to fumigate the greenhouse late autumn/early spring