Solution required - small, shrivelling leaves on runner beans

This year I used left over seed (Scarlet Emperor) and planted 6 beans in a wooden trough which I usually use for geraniums.  I scrubbed the liner before planting.  I have positioned the trough against the side of a open style trellis pergola so the beans can climb.  The lower half of the bean plants are as I would expect them to be - nice and green and large leaves.  The flowers are forming and there are signs of tiny beans.  I water when the earth feels dry an inch or so down and feed once a week with liquid tomato feed.  I mist the flowers most evenings - especially when it's been hot.  They are growing up the trellis well - except that the upper leaves seem to be a darker green, a lot smaller, shrivelled and nibbled on the edges.  I have inspected the underside of the leaves, but can't see any bugs - unless they are so tiny I've missed them!  There is a small amount of black fly on the lower leaves, but the ants seem to be taking care of those.  I didn't have this problem last year when I planted up a wigwam and had a great crop.  Any ideas what's causing the new leaves to be so small and wrinkled? the trough itself is in semi shade all day, but the climbers are in the full sun from about 11am onwards.  Thanks.