Tomato plants

[hardy49]

We have just noticed that,whilst they are growing well,the lower leaves on our tomato plants are curling up and dying.Also,some of them have brown markings on the leaves.We know the lower leaves do die off but not usually in this way.They are only watered with rain water from our water butts so could anybody have any ideas please.Hopefully the tomatoes will not be affected as we do have some flowers forming.