Plumtree

When did you plant the plum tree, mdjlucan ? 

Plums can be prone to magnesium deficiency. Which can cause yellowing between the leaf veins.

Have you used a high-potassium fertilisers, such as tomato feed ?  This can cause a magnesium deficiency as potassium is taken up in preference to magnesium.

The horse manure is a good idea. You could use a top dressing of chicken pellets.