tomato plant problem

Hi, I hope someone here might be able to help me with a problem I have with my tomato plants. I currently have 15 plants which I started off in early February. I have been growing these in a polytunnel tunnel and they seemed to be doing really well until two weeks ago. The lower leaves of the plants have curled upwards and are very brittle and easy to tear little paper. The new growth on the plant is dropping. They seem to have stopped growing any higher and I am really worried they are going to die after all the work I put in.When I first noticed the problem I taught large temperature fluctuations from day to day inside the polytunnel might be to blame but now I am worried it is something much worse. I have attached some pics so you might get a better idea what I am talking about.Any ideas what the problem might me? Thanks in advance