Yellow leaves on runner beans

My runner bean plants seem to have lots of yellowing leaves that wither and then die away completely. The plants are producing lots of lovely healthy beans (that taste yummy!). Is the plant in need of some nutrient or does it have a disease?

 

If it`s the lower leaves this is quite normal, if it`s not, then we need a pic to see the damage. As long as the beans are growing away alright I wouldn`t worry too much about the leaves.:gnthb:

 

Some of the leaves on some of my beans are turning yellow. I grew far too many plants initially, ran out of garden and stuck the remaining dozen or so in troughs and pots on a 'bung them in and see' basis. Mine (in the tubs) are turning yellow because I seldom remember to feed them.