It might be anthracnose.

What do the leaves and stems of the plants look like? Are there dark spots and blotches on them too? Have the plants lost a lot of foliage?

Here are some pics of tomato anthracnose:

Here is a link that might be useful: Anthracnose

Oh no!!! Yes - a few varities have lost a lot of their leaves. I got advice here earlier and it was suspected that it was Septoria leaf spot.

Now what do i do? I know it's too late for this season, but how can i prevent it for next season?