Is This Blight??

Of my 4 tomato plants, 2 are sick! The leaves have shrivelled and died, quite suddenly. They curled down and withered, and all the small fruits dropped off. A third plant is recovering slowly and a fourth, a more compact variety has only slight signs of up-curled leaves. They are well-watered in a shaded greenhouse which gets full sun. I have removed the sick ones from the greenhouse. Is this a disease? I didn't see any pests on them and there were no obvious markings on the leaves. I also have 20 potato plants in the veg patch, of which one is showing signs of the same problem - leaves curling at the edges. Can I spray them with something or will they go the same way as the toms??

 

Thank you, John, for your reply. Read the useful article you recommended - You may well be right about the overheating - I've moved all the toms into a sheltered spot outside, keeping the sick ones well apart, although I don't suppose there's any help for them - they may be too far gone to recover. As a beginner (first year) to veg growing, I'm very grateful for any advice. I have read with interest the posts about infected compost. I have not put anything on the new patch where I'm growing pots, dwarf French beans, cabbages and sprouts. All the plants are doing well except for this one potato plant, which is going pale with curling leaves (upcurl). Again no sign of pests or blotchiness / discolouration - just a drying out of the leaves. Its neighbours are fine. Any ideas?