Why havve my runner bean plants turned yellow?

Sounds very much like nitrogen deficiency. This is often caused when the soil contains organic matter that is insufficiently decomposed, such as the manure you have used. The process of decomposition uses up nitrogen that would normally be available to the plant. For treatment I am quoting from Collins Guide to Pests Diseases and Disorders of Garden Plants, p.474.
Apply top-dressings of nitrogen-containing fertilisers, such as ammonium sulphate, at or before planting and periodically thereafter according to mfrs. instructions. Adequate nitrogen is commonly supplied with phosphorus and potassium in balanced NPK fertilisers, but it should be remembered that nitrogen, unlike phosphorus and potassium, is readily leached from the soil, and fresh applications must be made for each crop. End quote.
I suggest you water on a feed that is high in nitrogen, making sure it gets on the leaves, as broad-leaved plants such as beans will absorb foliar feed.
Phostrogen should do the trick, applied at regular intervals. Don't use Miraclegro for this problem. It's too high in Potassium. Hope this helps.
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