Cucumber not thriving

This is not really a problem as I have 2 cucumbers in the fridge and there are still 4 growing slowly on the plant but the leaves on the main stem turned pale green then yellow and now are dead. There is a cucumber near the top and a few green leaves at the end still may be growing. 2 other vines (same plant) are on the floor with 3 smallish fruit still growing and some leaves on them, again some have yellowed. This picture was taken 2 weeks ago and most of the leaves in the picture are now dead:

   

Any ideas what the cause is likely to be (ignore the nettles in the background)?