problem with leaves on tomato plants

I have a plastic greenhouse and I am growing, aubergine, melon, cucumber, chillies and tomatoes (big boy, moneymaker) I bought some of the tomato plants from a garden centre and others I was given. The leaves on my tomatoes are beginning to go yellow at the ends, this gardaully creeps up the leaves to the stem of the plant, they then turn brown and detach from the stem. There are lots of flowers but no fruit forming, the flowers are just falling off. What is it and how do I resolve this problem?