Hope this helps...
Curling leaves can be the start of several different tomato diseases: Curly Top, Mosaic and Fusarium all start this way. However, if no additional symptoms appear, you simply have a case of Tomato Leaf Roll. This occurs more characteristically with certain varieties (Big Boy, for example) and although it is somewhat a mystery, it usually shows up after excessive rainfall or over watering. When the soil stays wet for too long, the older leaves on the lower half of the plant start to curl (roll) upward. In a few days, after the soil dries out, it may just go away. It's also thought that this condition can be caused by intense sunlight, which causes the accumulation of carbohydrates in the plant. It doesn't seem to have an adverse affect on yields, so unless other symptoms follow, it's nothing to worry about. Even feeding and watering-about 1 inch per week (more in hotter parts of the country) will produce the best yields. Water in the morning and from the bottom of the plant to keep leaves dry and help prevent disease.
I wonder if the plants on the other side of your polytunnel get more sunlight,than the side the toms are on ? thus your toms are not drying out as quickly...
I know I learned something from reading into this, I water mine daily in the morning, theyre in the greenhouse, but over the last few days its not been so sunny here and the soil has still been wet looking when i get in from work, will need to keep an eye on this, as I dont want loads of leaves.
This is the link to the info
http://www.thriftyfun.com/tf171119.tip.html